feat(channel): page details chaine multi-providers avec onglets et infinite scroll

- Strategy Pattern front: ChannelProviderInterface + 6 providers (yt/dm/tw/pt/od/ru) + factory (ids longs/courts)
- ChannelContentService: cache par onglet/tri/recherche (TTL 5min), pagination nextPage
- ChannelPage: header (banniere/avatar/subs/description ...plus), tabs dynamiques par capabilities, grilles videos/shorts 9:16/playlists/live, recherche debounced, tri recent/populaire, infinite scroll IntersectionObserver, skeletons/empty/error
- Backend: GET /api/channels/:provider/:id/content (videos/shorts/playlists/live) natif par provider

db/migrations/20250923_add_subscriptions_tables.sql

@@ -0,0 +1,27 @@
+BEGIN TRANSACTION;
+
+CREATE TABLE IF NOT EXISTS channels (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  provider TEXT NOT NULL,
+  external_id TEXT NOT NULL,
+  title TEXT,
+  handle TEXT,
+  avatar_url TEXT,
+  url TEXT,
+  subs_count INTEGER,
+  verified INTEGER DEFAULT 0,
+  last_refreshed_at INTEGER,
+  UNIQUE(provider, external_id)
+);
+
+CREATE TABLE IF NOT EXISTS subscriptions (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id INTEGER NOT NULL,
+  channel_id INTEGER NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+  created_at INTEGER NOT NULL,
+  UNIQUE(user_id, channel_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_subs_user ON subscriptions(user_id);
+
+COMMIT;