feat(M002): smart playlists — rule engine, editor UI, sidebar integration

Recovered from orphaned worktree commits (complete-milestone failed to merge).

Backend:
- Migration 9: is_smart + smart_rules columns on playlists table
- smartplaylist package: parameterized WHERE clause builder, field whitelist, genre subquery
- playlist.Service: Create/Update/Evaluate/Preview/GetRules smart playlist methods
- 65 tests (49 rule engine + 15 service + 1 migration)

Frontend:
- yj-combobox: reusable typeable dropdown with keyboard nav, ARIA, blur-race fix
- smart-playlist-editor: row-based rule builder with live preview
- smart-playlist-details: evaluate, refresh, play, shuffle, edit rules
- Sidebar: filter icon, Smart badge, create button, routing
- Queue snapshot on play/shuffle

backend/database/sql/schemas/playlists.sql

@@ -1,6 +1,8 @@
 CREATE TABLE IF NOT EXISTS playlists (
     id INTEGER PRIMARY KEY,
     name TEXT NOT NULL,
+    is_smart INTEGER NOT NULL DEFAULT 0,
+    smart_rules TEXT,
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );

backend/database/sql/sqlcgen/models.go

@@ -73,10 +73,12 @@ type PlayerState struct {
 }
 
 type Playlist struct {
-	ID        int64
-	Name      string
-	CreatedAt time.Time
-	UpdatedAt time.Time
+	ID         int64
+	Name       string
+	IsSmart    int64
+	SmartRules sql.NullString
+	CreatedAt  time.Time
+	UpdatedAt  time.Time
 }
 
 type PlaylistTrack struct {

backend/database/sql/sqlcgen/playlists.sql.go

@@ -82,7 +82,7 @@ func (q *Queries) CountPlaylistsByName(ctx context.Context, name string) (int64,
 
 const createPlaylist = `-- name: CreatePlaylist :one
 INSERT INTO playlists (name) VALUES (?)
-RETURNING id, name, created_at, updated_at
+RETURNING id, name, is_smart, smart_rules, created_at, updated_at
 `
 
 func (q *Queries) CreatePlaylist(ctx context.Context, name string) (Playlist, error) {
@@ -91,6 +91,8 @@ func (q *Queries) CreatePlaylist(ctx context.Context, name string) (Playlist, er
 	err := row.Scan(
 		&i.ID,
 		&i.Name,
+		&i.IsSmart,
+		&i.SmartRules,
 		&i.CreatedAt,
 		&i.UpdatedAt,
 	)
@@ -192,7 +194,7 @@ func (q *Queries) GetAllPlaylistTracksWithMetadata(ctx context.Context) ([]GetAl
 }
 
 const getAllPlaylists = `-- name: GetAllPlaylists :many
-SELECT id, name, created_at, updated_at FROM playlists ORDER BY updated_at DESC
+SELECT id, name, is_smart, smart_rules, created_at, updated_at FROM playlists ORDER BY updated_at DESC
 `
 
 func (q *Queries) GetAllPlaylists(ctx context.Context) ([]Playlist, error) {
@@ -207,6 +209,8 @@ func (q *Queries) GetAllPlaylists(ctx context.Context) ([]Playlist, error) {
 		if err := rows.Scan(
 			&i.ID,
 			&i.Name,
+			&i.IsSmart,
+			&i.SmartRules,
 			&i.CreatedAt,
 			&i.UpdatedAt,
 		); err != nil {
@@ -236,7 +240,7 @@ func (q *Queries) GetNextPlaylistTrackPosition(ctx context.Context, playlistID i
 }
 
 const getPlaylist = `-- name: GetPlaylist :one
-SELECT id, name, created_at, updated_at FROM playlists WHERE id = ? LIMIT 1
+SELECT id, name, is_smart, smart_rules, created_at, updated_at FROM playlists WHERE id = ? LIMIT 1
 `
 
 func (q *Queries) GetPlaylist(ctx context.Context, id int64) (Playlist, error) {
@@ -245,6 +249,8 @@ func (q *Queries) GetPlaylist(ctx context.Context, id int64) (Playlist, error) {
 	err := row.Scan(
 		&i.ID,
 		&i.Name,
+		&i.IsSmart,
+		&i.SmartRules,
 		&i.CreatedAt,
 		&i.UpdatedAt,
 	)