feat(db): add contextual_likes table (M3 session-context capture)

Table was referenced in migration 0005's comment but never created —
this slice fills the gap. Soft-delete via deleted_at column; hot-path
partial index on active rows; GIN index on session_vector for
M3 sub-plan #3's similarity queries.

internal/db/dbq/contextual_likes.sql.go

@@ -0,0 +1,52 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: contextual_likes.sql
+
+package dbq
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+const insertContextualLike = `-- name: InsertContextualLike :exec
+INSERT INTO contextual_likes (user_id, track_id, session_vector, session_id)
+VALUES ($1, $2, $3, $4)
+`
+
+type InsertContextualLikeParams struct {
+	UserID        pgtype.UUID
+	TrackID       pgtype.UUID
+	SessionVector []byte
+	SessionID     pgtype.UUID
+}
+
+func (q *Queries) InsertContextualLike(ctx context.Context, arg InsertContextualLikeParams) error {
+	_, err := q.db.Exec(ctx, insertContextualLike,
+		arg.UserID,
+		arg.TrackID,
+		arg.SessionVector,
+		arg.SessionID,
+	)
+	return err
+}
+
+const softDeleteContextualLikesForUserTrack = `-- name: SoftDeleteContextualLikesForUserTrack :exec
+UPDATE contextual_likes
+SET deleted_at = now()
+WHERE user_id = $1 AND track_id = $2 AND deleted_at IS NULL
+`
+
+type SoftDeleteContextualLikesForUserTrackParams struct {
+	UserID  pgtype.UUID
+	TrackID pgtype.UUID
+}
+
+// Marks all currently-active rows for (user, track) as deleted. Idempotent —
+// already-deleted rows aren't re-touched.
+func (q *Queries) SoftDeleteContextualLikesForUserTrack(ctx context.Context, arg SoftDeleteContextualLikesForUserTrackParams) error {
+	_, err := q.db.Exec(ctx, softDeleteContextualLikesForUserTrack, arg.UserID, arg.TrackID)
+	return err
+}

internal/db/dbq/models.go

@@ -29,6 +29,16 @@ type Artist struct {
 	UpdatedAt pgtype.Timestamptz
 }
 
+type ContextualLike struct {
+	ID            pgtype.UUID
+	UserID        pgtype.UUID
+	TrackID       pgtype.UUID
+	LikedAt       pgtype.Timestamptz
+	DeletedAt     pgtype.Timestamptz
+	SessionVector []byte
+	SessionID     pgtype.UUID
+}
+
 type GeneralLike struct {
 	UserID  pgtype.UUID
 	TrackID pgtype.UUID