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/nonces.sql

@@ -0,0 +1,22 @@
+-- Table: public.nonces
+
+-- DROP TABLE public.nonces;
+
+CREATE TABLE IF NOT EXISTS public.nonces
+(
+  nonce text,
+  expire timestamp with time zone,
+  CONSTRAINT nonces_id_key UNIQUE (nonce)
+);
+
+GRANT ALL ON TABLE public.nonces TO current_user;
+
+-- Index: public.nonces_nonce_idx
+
+-- DROP INDEX public.nonces_nonce_idx;
+
+CREATE INDEX IF NOT EXISTS nonces_nonce_idx
+  ON public.nonces
+  USING btree
+  (nonce COLLATE pg_catalog."default");
+