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/channel_videos.sql

@@ -0,0 +1,30 @@
+-- Table: public.channel_videos
+
+-- DROP TABLE public.channel_videos;
+
+CREATE TABLE IF NOT EXISTS public.channel_videos
+(
+  id text NOT NULL,
+  title text,
+  published timestamp with time zone,
+  updated timestamp with time zone,
+  ucid text,
+  author text,
+  length_seconds integer,
+  live_now boolean,
+  premiere_timestamp timestamp with time zone,
+  views bigint,
+  CONSTRAINT channel_videos_id_key UNIQUE (id)
+);
+
+GRANT ALL ON TABLE public.channel_videos TO current_user;
+
+-- Index: public.channel_videos_ucid_idx
+
+-- DROP INDEX public.channel_videos_ucid_idx;
+
+CREATE INDEX IF NOT EXISTS channel_videos_ucid_idx
+  ON public.channel_videos
+  USING btree
+  (ucid COLLATE pg_catalog."default");
+