feat(taste): phase 1 — persistent per-user taste profile from graded engagement (#796)

Build a persistent, decaying model of each user's taste, recomputed daily,
that later phases consume across every recommendation surface. Phase 1 only
BUILDS the object — no behaviour change to what's surfaced yet.

Core mechanic — graded engagement (replaces binary was_skipped for learning;
was_skipped stays for History): a play's completion ratio maps to a signal in
[-1,+1] via two linear ramps (instant-skip → -1, ~0.30 neutral, ≥0.90 → +1).
Time-decayed (half-life ~75d) so recent behaviour dominates and the profile
tracks drift.

Per operator constraints:
- No explicit dislike button — negatives come only from passive behaviour
  (early skips). Nothing recorded to regret or opt out of.
- Negatives are track-scoped; artist/tag weight is the decayed SUM of their
  tracks' engagement, so one skip nets out against many good plays (a
  DB test asserts a liked artist stays positive despite an early-skipped
  track). A floor clamp bounds how negative any single entity can get.

- migration 0035: taste_profile_artists / taste_profile_tags (signed weight,
  indexed by (user, weight DESC)).
- internal/taste: engagement.go (pure curve + decay) + profile.go
  (accumulate plays + like bonuses, floor damping, size caps, atomic-replace).
- scheduler: rebuildUserDaily recomputes the profile before the playlist
  build (so phase 2 can read it), best-effort — a taste failure never blocks
  playlist building. Wired into the daily job + startup catch-up only (not
  manual/lazy rebuilds).
- tests: pure (engagement curve, decay, ranking, floor, genre split) +
  DB-backed (positive/negative weights, aggregation-protects-artist, like
  bonus, atomic replace). All green vs real Postgres.

Config knobs live in taste.DefaultConfig() for now; wiring them into the
server RecommendationConfig is a later follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/playlists/scheduler.go

@@ -28,6 +28,7 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/taste"
 )
 
 // dailyRebuildHour is the local-time hour at which each user's system
@@ -158,10 +159,7 @@ func (s *Scheduler) registerLocked(ctx context.Context, userID pgtype.UUID, tz s
 	job, err := s.gocron.NewJob(
 		gocron.CronJob(cronExpr, false /* withSeconds */),
 		gocron.NewTask(func() {
-			if err := BuildSystemPlaylists(ctx, s.pool, s.logger, userID, time.Now(), s.dataDir); err != nil {
-				s.logger.Warn("scheduler: build failed",
-					"user_id", uuidStringPL(userID), "err", err)
-			}
+			s.rebuildUserDaily(ctx, userID, time.Now())
 		}),
 	)
 	if err != nil {
@@ -220,10 +218,22 @@ func (s *Scheduler) reconcile(ctx context.Context) {
 // whose scheduled fire was missed during downtime get caught up.
 func (s *Scheduler) runStartupCatchUp(ctx context.Context, users []dbq.ListActiveUsersWithTimezonesRow) {
 	for _, u := range users {
-		if err := BuildSystemPlaylists(ctx, s.pool, s.logger, u.ID, time.Now(), s.dataDir); err != nil {
-			s.logger.Warn("scheduler: startup catch-up build failed",
-				"user_id", uuidStringPL(u.ID), "err", err)
-		}
+		s.rebuildUserDaily(ctx, u.ID, time.Now())
+	}
+}
+
+// rebuildUserDaily recomputes the user's taste profile, then rebuilds their
+// system playlists + You-might-like rows. Taste runs first so the playlist
+// build can read a fresh profile (phase-2 consumption); both steps are
+// best-effort and a failure in one is logged without blocking the other.
+func (s *Scheduler) rebuildUserDaily(ctx context.Context, userID pgtype.UUID, now time.Time) {
+	if err := taste.BuildTasteProfile(ctx, s.pool, s.logger, userID, now, taste.DefaultConfig()); err != nil {
+		s.logger.Warn("scheduler: taste profile rebuild failed",
+			"user_id", uuidStringPL(userID), "err", err)
+	}
+	if err := BuildSystemPlaylists(ctx, s.pool, s.logger, userID, now, s.dataDir); err != nil {
+		s.logger.Warn("scheduler: build failed",
+			"user_id", uuidStringPL(userID), "err", err)
 	}
 }