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/playlists.sql

@@ -0,0 +1,29 @@
+-- Type: public.privacy
+
+-- DROP TYPE public.privacy;
+
+CREATE TYPE public.privacy AS ENUM
+(
+    'Public',
+    'Unlisted',
+    'Private'
+);
+
+-- Table: public.playlists
+
+-- DROP TABLE public.playlists;
+
+CREATE TABLE IF NOT EXISTS public.playlists
+(
+    title text,
+    id text primary key,
+    author text,
+    description text,
+    video_count integer,
+    created timestamptz,
+    updated timestamptz,
+    privacy privacy,
+    index int8[]
+);
+
+GRANT ALL ON public.playlists TO current_user;