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_test.go

@@ -2,6 +2,7 @@ package playevents
 
 import (
 	"context"
+	"encoding/json"
 	"io"
 	"log/slog"
 	"os"
@@ -234,3 +235,85 @@ func TestRecordSyntheticCompletedPlay_WritesBothRows(t *testing.T) {
 		t.Errorf("closed play_events count = %d, want 1", count)
 	}
 }
+
+func TestRecordPlayStarted_PersistsSessionVector_Seed(t *testing.T) {
+	f := newFixture(t, 200_000)
+	now := time.Now().UTC()
+	res, err := f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", now)
+	if err != nil {
+		t.Fatalf("RecordPlayStarted: %v", err)
+	}
+	got, err := f.q.GetPlayEventByID(context.Background(), res.PlayEventID)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got.SessionVectorAtPlay == nil {
+		t.Fatalf("session_vector_at_play is NULL")
+	}
+	var vec map[string]any
+	if err := json.Unmarshal(got.SessionVectorAtPlay, &vec); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if seed, _ := vec["seed"].(bool); !seed {
+		t.Errorf("seed = %v, want true (first play in session)", vec["seed"])
+	}
+	if artists, _ := vec["artists"].([]any); len(artists) != 0 {
+		t.Errorf("artists not empty: %v", artists)
+	}
+}
+
+func TestRecordPlayStarted_PersistsSessionVector_Populated(t *testing.T) {
+	f := newFixture(t, 200_000)
+	// Seed 4 prior plays with the same track (reusing the fixture's single
+	// track is fine — vector dedup means we'll see 1 artist regardless).
+	t0 := time.Now().UTC().Add(-1 * time.Hour)
+	for i := 0; i < 4; i++ {
+		at := t0.Add(time.Duration(i) * time.Minute)
+		_, _ = f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", at)
+	}
+	// 5th play — should see vector with 4 prior tracks.
+	at := t0.Add(10 * time.Minute)
+	res, err := f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", at)
+	if err != nil {
+		t.Fatalf("RecordPlayStarted: %v", err)
+	}
+	got, _ := f.q.GetPlayEventByID(context.Background(), res.PlayEventID)
+	if got.SessionVectorAtPlay == nil {
+		t.Fatal("session_vector_at_play is NULL")
+	}
+	var vec map[string]any
+	_ = json.Unmarshal(got.SessionVectorAtPlay, &vec)
+	if seed, _ := vec["seed"].(bool); seed {
+		t.Errorf("seed = true after 4 prior tracks, want false")
+	}
+	recentIDs, _ := vec["recent_track_ids"].([]any)
+	if len(recentIDs) != 4 {
+		t.Errorf("recent_track_ids len = %d, want 4", len(recentIDs))
+	}
+}
+
+func TestRecordPlayStarted_VectorScopedToSession(t *testing.T) {
+	f := newFixture(t, 200_000)
+	// Play 1: at t=0.
+	t0 := time.Now().UTC().Add(-2 * time.Hour)
+	_, _ = f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", t0)
+	// Play 2: t = t0 + 31 minutes (exceeds 30-min session timeout).
+	t1 := t0.Add(31 * time.Minute)
+	res, err := f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", t1)
+	if err != nil {
+		t.Fatalf("RecordPlayStarted: %v", err)
+	}
+	got, _ := f.q.GetPlayEventByID(context.Background(), res.PlayEventID)
+	if got.SessionVectorAtPlay == nil {
+		t.Fatal("vector NULL")
+	}
+	var vec map[string]any
+	_ = json.Unmarshal(got.SessionVectorAtPlay, &vec)
+	if seed, _ := vec["seed"].(bool); !seed {
+		t.Errorf("seed = false in fresh session, want true")
+	}
+	recentIDs, _ := vec["recent_track_ids"].([]any)
+	if len(recentIDs) != 0 {
+		t.Errorf("recent_track_ids has %d entries from prior session, want 0", len(recentIDs))
+	}
+}