feat(playevents): persist session_vector_at_play on every record path

RecordPlayStarted and RecordSyntheticCompletedPlay both capture the
session vector inside their existing transactions. Single private
helper queries the prior 5 tracks, builds the vector, UPDATEs the
just-inserted play_event. Tests verify the seed flag boundary and
session-scope isolation.

internal/playevents/writer.go

@@ -9,6 +9,7 @@ package playevents
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"log/slog"
 	"time"
@@ -19,6 +20,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/playsessions"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/recommendation"
 )
 
 type Writer struct {
@@ -87,6 +89,11 @@ func (w *Writer) RecordPlayStarted(
 		}
 		out.PlayEventID = ev.ID
 		out.SessionID = sessionID
+
+		// Capture session vector for the just-inserted row.
+		if err := w.captureSessionVector(ctx, q, ev.ID, sessionID, at); err != nil {
+			return err
+		}
 		return nil
 	})
 	return out, err
@@ -223,6 +230,9 @@ func (w *Writer) RecordSyntheticCompletedPlay(
 		if err != nil {
 			return err
 		}
+		if err := w.captureSessionVector(ctx, q, ev.ID, sessionID, at); err != nil {
+			return err
+		}
 		ratio := 1.0
 		dur := track.DurationMs
 		_, err = q.UpdatePlayEventEnded(ctx, dbq.UpdatePlayEventEndedParams{
@@ -236,6 +246,36 @@ func (w *Writer) RecordSyntheticCompletedPlay(
 	})
 }
 
+// captureSessionVector queries the user's prior plays in the given session
+// (before `at`), builds the session vector, and UPDATEs the just-inserted
+// play_event with it. Runs inside the caller's transaction (q is the
+// transaction-bound *dbq.Queries). Errors here propagate — vector capture
+// is best-effort, but DB errors should fail the transaction.
+func (w *Writer) captureSessionVector(
+	ctx context.Context,
+	q *dbq.Queries,
+	playEventID, sessionID pgtype.UUID,
+	at time.Time,
+) error {
+	priorTracks, err := q.ListRecentSessionTracks(ctx, dbq.ListRecentSessionTracksParams{
+		SessionID: sessionID,
+		StartedAt: pgtype.Timestamptz{Time: at, Valid: true},
+		Limit:     5,
+	})
+	if err != nil {
+		return err
+	}
+	vec := recommendation.BuildSessionVector(priorTracks)
+	vecJSON, err := json.Marshal(vec)
+	if err != nil {
+		return err
+	}
+	return q.UpdatePlayEventVector(ctx, dbq.UpdatePlayEventVectorParams{
+		ID:                  playEventID,
+		SessionVectorAtPlay: vecJSON,
+	})
+}
+
 // autoClosePriorOpen closes any open (ended_at IS NULL) play_event for the
 // user. Sets ended_at = at, duration_played_ms = min(at - started_at, track
 // duration), was_skipped = true. Skip rule is NOT applied — auto-closed