feat(api): radio handler reads current session vector + threads ContextWeight

Wire contextual scoring end-to-end: add ContextWeight to RecommendationConfig
(struct + Default()), fetch the user's current session vector in handleRadio
via loadCurrentSessionVector (cold-start returns Seed sentinel), and pass it
as the 6th arg to LoadCandidates so ContextualMatchScore flows into Shuffle.

internal/api/auth_test.go

@@ -52,7 +52,7 @@ func testHandlers(t *testing.T) (*handlers, *pgxpool.Pool) {
 	w := playevents.NewWriter(pool, logger, 30*time.Minute, 0.5, 30000)
 	recCfg := config.RecommendationConfig{
 		BaseWeight: 1.0, LikeBoost: 2.0, RecencyWeight: 1.0,
-		SkipPenalty: 1.0, JitterMagnitude: 0.1,
+		SkipPenalty: 1.0, JitterMagnitude: 0.1, ContextWeight: 2.0,
 		RecentlyPlayedHours: 1, RadioSize: 50, RadioSizeMax: 200,
 	}
 	h := &handlers{pool: pool, logger: logger, events: w, recCfg: recCfg, rng: func() float64 { return 0.5 }}

internal/api/radio.go

@@ -1,13 +1,16 @@
 package api
 
 import (
+	"encoding/json"
 	"errors"
+	"log/slog"
 	"net/http"
 	"strconv"
 	"strings"
 	"time"
 
 	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
@@ -22,7 +25,9 @@ type RadioResponse struct {
 // handleRadio implements GET /api/radio?seed_track=<uuid>&limit=<int>.
 //
 // Returns the seed at index 0, followed by up to limit-1 weighted-shuffle
-// picks from the user's library, scored by recommendation.Score.
+// picks from the user's library, scored by recommendation.Score. The
+// scoring formula folds in contextual_match_score using the user's current
+// session vector (read from the most recent open play_event).
 func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
 	user, ok := auth.UserFromContext(r.Context())
 	if !ok {
@@ -74,7 +79,10 @@ func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
 		return
 	}
-	candidates, err := recommendation.LoadCandidates(r.Context(), q, user.ID, seedID, h.recCfg.RecentlyPlayedHours)
+
+	currentVec := loadCurrentSessionVector(r, q, user.ID, h.logger)
+
+	candidates, err := recommendation.LoadCandidates(r.Context(), q, user.ID, seedID, h.recCfg.RecentlyPlayedHours, currentVec)
 	if err != nil {
 		h.logger.Error("api: radio: load candidates", "err", err)
 		writeErr(w, http.StatusInternalServerError, "server_error", "candidate load failed")
@@ -86,6 +94,7 @@ func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
 		RecencyWeight:   h.recCfg.RecencyWeight,
 		SkipPenalty:     h.recCfg.SkipPenalty,
 		JitterMagnitude: h.recCfg.JitterMagnitude,
+		ContextWeight:   h.recCfg.ContextWeight,
 	}
 	picks := recommendation.Shuffle(candidates, weights, time.Now().UTC(), h.rng, limit-1)
 
@@ -108,3 +117,27 @@ func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
 	}
 	writeJSON(w, http.StatusOK, RadioResponse{Tracks: out})
 }
+
+// loadCurrentSessionVector returns the user's most recent active session
+// vector, or a Seed=true sentinel if none exists / the column is NULL /
+// the JSON fails to unmarshal. Sentinel short-circuits ContextualMatchScore
+// to 0 so the contextual term contributes nothing in cold-start cases.
+func loadCurrentSessionVector(r *http.Request, q *dbq.Queries, userID pgtype.UUID, logger *slog.Logger) recommendation.SessionVector {
+	raw, err := q.GetCurrentSessionVectorForUser(r.Context(), userID)
+	if err != nil {
+		// pgx.ErrNoRows is the common path: no active session yet.
+		if !errors.Is(err, pgx.ErrNoRows) {
+			logger.Warn("api: radio: load current session vector", "err", err)
+		}
+		return recommendation.SessionVector{Seed: true}
+	}
+	if len(raw) == 0 {
+		return recommendation.SessionVector{Seed: true}
+	}
+	var v recommendation.SessionVector
+	if jerr := json.Unmarshal(raw, &v); jerr != nil {
+		logger.Warn("api: radio: bad session_vector_at_play json", "err", jerr)
+		return recommendation.SessionVector{Seed: true}
+	}
+	return v
+}

internal/config/config.go

@@ -72,6 +72,7 @@ type RecommendationConfig struct {
 	RecencyWeight       float64 `yaml:"recency_weight"`
 	SkipPenalty         float64 `yaml:"skip_penalty"`
 	JitterMagnitude     float64 `yaml:"jitter_magnitude"`
+	ContextWeight       float64 `yaml:"context_weight"`
 	RecentlyPlayedHours int     `yaml:"recently_played_hours"`
 	RadioSize           int     `yaml:"radio_size"`
 	RadioSizeMax        int     `yaml:"radio_size_max"`
@@ -93,6 +94,7 @@ func Default() Config {
 			RecencyWeight:       1.0,
 			SkipPenalty:         1.0,
 			JitterMagnitude:     0.1,
+			ContextWeight:       2.0,
 			RecentlyPlayedHours: 1,
 			RadioSize:           50,
 			RadioSizeMax:        200,