feat: nginx reverse proxy, Spotify import, overlay system, UI overhaul

- Add nginx as single entry point: /api/* → backend, /* → web
- NEXT_PUBLIC_API_URL="" so all API calls are relative (go through nginx)
- Add Spotify playlist import (Client Credentials OAuth, up to 500 tracks)
- Add Yandex/Spotify tabbed import UI on /playlists
- Add stream overlay system (SSE + polling fallback, 9 styles)
- Reorganize pages into (main) route group
- Add QueuePanel, VersionsPanel, Toaster components
- Add overlay settings tab in /settings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/backend/internal/models/models.go

@@ -52,11 +52,11 @@ type User struct {
 
 type Playlist struct {
 	ID        string          `gorm:"primaryKey;type:varchar(36)" json:"id"`
-	UserID    string          `gorm:"not null;index" json:"user_id"`
+	UserID    string          `gorm:"not null;index;index:idx_playlist_user_created,composite:1" json:"user_id"`
 	Name      string          `gorm:"not null" json:"name"`
-	IsPublic  bool            `gorm:"default:false" json:"is_public"`
+	IsPublic  bool            `gorm:"default:false;index:idx_playlist_public_created,composite:1" json:"is_public"`
 	Tags      string          `gorm:"default:''" json:"-"`
-	CreatedAt time.Time       `json:"created_at"`
+	CreatedAt time.Time       `gorm:"index:idx_playlist_public_created,composite:2;index:idx_playlist_user_created,composite:2" json:"created_at"`
 	Tracks    []PlaylistTrack `gorm:"foreignKey:PlaylistID" json:"tracks,omitempty"`
 }