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/annotations.sql

@@ -0,0 +1,12 @@
+-- Table: public.annotations
+
+-- DROP TABLE public.annotations;
+
+CREATE TABLE IF NOT EXISTS public.annotations
+(
+  id text NOT NULL,
+  annotations xml,
+  CONSTRAINT annotations_id_key UNIQUE (id)
+);
+
+GRANT ALL ON TABLE public.annotations TO current_user;