Update invidious stack to use companion approach

- Replace inv_sig_helper with invidious-companion for better YouTube API handling
- Add healthcheck for main container
- Add resource limits for all containers
- Add SQL init scripts for fresh database setup
- Update README with invidious secrets documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

stacks/invidious/config/sql/playlist_videos.sql

@@ -0,0 +1,19 @@
+-- Table: public.playlist_videos
+
+-- DROP TABLE public.playlist_videos;
+
+CREATE TABLE IF NOT EXISTS public.playlist_videos
+(
+    title text,
+    id text,
+    author text,
+    ucid text,
+    length_seconds integer,
+    published timestamptz,
+    plid text references playlists(id),
+    index int8,
+    live_now boolean,
+    PRIMARY KEY (index,plid)
+);
+
+GRANT ALL ON TABLE public.playlist_videos TO current_user;