feat(metrics): bucketed surface families + manual-plays baseline (#1248, milestone 127)

The recommendation metrics table was observable but not actionable: raw
source strings (album:<uuid> one-offs) drowned the stable surfaces, and
manual plays were excluded so skip rates had no control group.

- SQL: include NULL-source rows (the baseline) and carry completion_n
  so family merges can weight avg_completion correctly.
- Handler buckets raw sources into stable families (radio:<uuid> →
  Radio, album:/artist: → direct plays, etc.) grouped by surface
  intent: go-to / discovery / direct — each band judged against its
  job, since discovery mixes are expected to skip hotter. Families
  under 20 plays are flagged low-confidence, not hidden.
- Settings card renders the baseline row and per-surface deltas in
  percentage points vs baseline (worse-than-baseline deltas in danger
  color), intent hint copy per group, low-data rows dimmed.
- Pure-unit test for the bucketing/merge; DB test updated to the new
  contract (baseline included, radio:<uuid> collapse).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

internal/api/me_recommendation_metrics.go

@@ -2,7 +2,9 @@ package api
 
 import (
 	"net/http"
+	"sort"
 	"strconv"
+	"strings"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
@@ -11,28 +13,135 @@ import (
 const (
 	recMetricsDefaultDays = 30
 	recMetricsMaxDays     = 365
+
+	// recMetricsLowVolume marks a family as low-confidence rather than
+	// hiding it: with fewer plays than this a skip rate is anecdote, not
+	// signal, but silently dropping the row would misread as "surface
+	// unused". The web renders low-confidence rows dimmed.
+	recMetricsLowVolume = 20
 )
 
-// recommendationMetric is one recommendation surface's outcomes.
-type recommendationMetric struct {
-	Source        string  `json:"source"`         // 'for_you' | 'discover' | mixes
-	Plays         int64   `json:"plays"`          // plays launched from this surface
+// Surface intents (milestone #127): each family is judged against its
+// job, not one global bar — discovery mixes are EXPECTED to run higher
+// skip rates than the go-to surfaces.
+const (
+	intentGoTo      = "go_to"
+	intentDiscovery = "discovery"
+	intentDirect    = "direct"
+)
+
+// surfaceMetric is one bucketed surface family's outcomes.
+type surfaceMetric struct {
+	Key           string  `json:"key"`            // stable family key ("for_you", "radio", …)
+	Label         string  `json:"label"`          // display label
+	Plays         int64   `json:"plays"`          // plays launched from this family
 	Skips         int64   `json:"skips"`          // of those, marked skipped
 	SkipRate      float64 `json:"skip_rate"`      // skips / plays, [0,1]
 	AvgCompletion float64 `json:"avg_completion"` // mean completion ratio, [0,1]
+	LowConfidence bool    `json:"low_confidence"` // plays < recMetricsLowVolume
+}
+
+// surfaceGroup is one intent band of surface families.
+type surfaceGroup struct {
+	Intent   string          `json:"intent"` // go_to | discovery | direct
+	Label    string          `json:"label"`
+	Surfaces []surfaceMetric `json:"surfaces"`
 }
 
 type recommendationMetricsResp struct {
-	WindowDays int                    `json:"window_days"`
-	Sources    []recommendationMetric `json:"sources"`
+	WindowDays int `json:"window_days"`
+	// Baseline is the control group: plays the user picked manually
+	// (source IS NULL). Surfaces are judged as deltas against it; nil
+	// when the window holds no manual plays.
+	Baseline *surfaceMetric `json:"baseline"`
+	Groups   []surfaceGroup `json:"groups"`
+}
+
+// recFamily is the bucketing target for a raw play_events.source value.
+type recFamily struct {
+	key    string
+	label  string
+	intent string
+}
+
+// bucketRecSource maps a raw client-stamped source string to its stable
+// family. One-off sources (album:<uuid>, radio:<uuid>) collapse into
+// their family so the table stays readable at any library size.
+func bucketRecSource(src string) recFamily {
+	switch {
+	case src == "for_you":
+		return recFamily{"for_you", "For You", intentGoTo}
+	case src == "songs_like_artist":
+		return recFamily{"songs_like_artist", "Songs like…", intentGoTo}
+	case src == "radio" || strings.HasPrefix(src, "radio:"):
+		return recFamily{"radio", "Radio", intentGoTo}
+	case src == "discover":
+		return recFamily{"discover", "Discover", intentDiscovery}
+	case src == "deep_cuts":
+		return recFamily{"deep_cuts", "Deep cuts", intentDiscovery}
+	case src == "rediscover":
+		return recFamily{"rediscover", "Rediscover", intentDiscovery}
+	case src == "new_for_you":
+		return recFamily{"new_for_you", "New for you", intentDiscovery}
+	case src == "on_this_day":
+		return recFamily{"on_this_day", "On this day", intentDiscovery}
+	case src == "first_listens":
+		return recFamily{"first_listens", "First listens", intentDiscovery}
+	case strings.HasPrefix(src, "album:"):
+		return recFamily{"direct_album", "Album plays", intentDirect}
+	case strings.HasPrefix(src, "artist:"):
+		return recFamily{"direct_artist", "Artist plays", intentDirect}
+	case strings.HasPrefix(src, "offline:"):
+		return recFamily{"offline", "Offline pools", intentDirect}
+	case strings.HasPrefix(src, "home:"):
+		return recFamily{"home", "Home sections", intentDirect}
+	case src == "history":
+		return recFamily{"history", "History", intentDirect}
+	default:
+		return recFamily{"other", "Other", intentDirect}
+	}
+}
+
+// familyAccum merges raw source rows into one family, carrying the
+// completion sample count so the merged average stays play-weighted.
+type familyAccum struct {
+	fam         recFamily
+	plays       int64
+	skips       int64
+	completionN int64
+	// completionSum is avg*count re-expanded, so merging N raw rows
+	// reduces to a single weighted division at the end.
+	completionSum float64
+}
+
+func (a *familyAccum) add(row dbq.RecommendationSourceMetricsForUserRow) {
+	a.plays += row.Plays
+	a.skips += row.Skips
+	a.completionN += row.CompletionN
+	a.completionSum += row.AvgCompletion * float64(row.CompletionN)
+}
+
+func (a *familyAccum) metric() surfaceMetric {
+	m := surfaceMetric{
+		Key:           a.fam.key,
+		Label:         a.fam.label,
+		Plays:         a.plays,
+		Skips:         a.skips,
+		LowConfidence: a.plays < recMetricsLowVolume,
+	}
+	if a.plays > 0 {
+		m.SkipRate = float64(a.skips) / float64(a.plays)
+	}
+	if a.completionN > 0 {
+		m.AvgCompletion = a.completionSum / float64(a.completionN)
+	}
+	return m
 }
 
 // handleGetRecommendationMetrics implements GET /api/me/recommendation-metrics.
-// Per-source play outcomes (plays / skips / skip-rate / avg-completion) for the
-// caller over the last `days` (default 30, capped at 365), so the operator can
-// see which recommendation surfaces are landing and tune the taste weights.
-// Only plays tagged with a system-playlist source count; library/radio plays
-// (no source) are excluded.
+// Bucketed per-surface-family outcomes for the caller over the last `days`
+// (default 30, capped at 365), grouped by surface intent and anchored by the
+// manual-plays baseline so the numbers are judgeable, not just observable.
 func (h *handlers) handleGetRecommendationMetrics(w http.ResponseWriter, r *http.Request) {
 	caller, ok := requireUser(w, r)
 	if !ok {
@@ -51,28 +160,59 @@ func (h *handlers) handleGetRecommendationMetrics(w http.ResponseWriter, r *http
 		return
 	}
 
-	out := recommendationMetricsResp{
-		WindowDays: days,
-		Sources:    make([]recommendationMetric, 0, len(rows)),
-	}
+	writeJSON(w, http.StatusOK, bucketMetricsResponse(days, rows))
+}
+
+// bucketMetricsResponse folds the raw per-source rows into the grouped,
+// baseline-anchored response shape. Split from the handler for pure-unit
+// testability.
+func bucketMetricsResponse(
+	days int, rows []dbq.RecommendationSourceMetricsForUserRow,
+) recommendationMetricsResp {
+	baseline := &familyAccum{fam: recFamily{"manual", "Manual library plays", ""}}
+	families := map[string]*familyAccum{}
 	for _, row := range rows {
-		source := ""
-		if row.Source != nil {
-			source = *row.Source
+		if row.Source == nil || *row.Source == "" {
+			baseline.add(row)
+			continue
 		}
-		var skipRate float64
-		if row.Plays > 0 {
-			skipRate = float64(row.Skips) / float64(row.Plays)
+		fam := bucketRecSource(*row.Source)
+		acc, exists := families[fam.key]
+		if !exists {
+			acc = &familyAccum{fam: fam}
+			families[fam.key] = acc
 		}
-		out.Sources = append(out.Sources, recommendationMetric{
-			Source:        source,
-			Plays:         row.Plays,
-			Skips:         row.Skips,
-			SkipRate:      skipRate,
-			AvgCompletion: row.AvgCompletion,
-		})
+		acc.add(row)
 	}
-	writeJSON(w, http.StatusOK, out)
+
+	resp := recommendationMetricsResp{WindowDays: days, Groups: []surfaceGroup{}}
+	if baseline.plays > 0 {
+		m := baseline.metric()
+		resp.Baseline = &m
+	}
+	for _, g := range []struct{ intent, label string }{
+		{intentGoTo, "Go-to surfaces"},
+		{intentDiscovery, "Discovery mixes"},
+		{intentDirect, "Direct plays"},
+	} {
+		group := surfaceGroup{Intent: g.intent, Label: g.label}
+		for _, acc := range families {
+			if acc.fam.intent == g.intent {
+				group.Surfaces = append(group.Surfaces, acc.metric())
+			}
+		}
+		if len(group.Surfaces) == 0 {
+			continue
+		}
+		sort.Slice(group.Surfaces, func(i, j int) bool {
+			if group.Surfaces[i].Plays != group.Surfaces[j].Plays {
+				return group.Surfaces[i].Plays > group.Surfaces[j].Plays
+			}
+			return group.Surfaces[i].Key < group.Surfaces[j].Key
+		})
+		resp.Groups = append(resp.Groups, group)
+	}
+	return resp
 }
 
 // parseMetricsDays reads the `days` query param (default 30, capped at 365).