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/users.sql

@@ -0,0 +1,29 @@
+-- Table: public.users
+
+-- DROP TABLE public.users;
+
+CREATE TABLE IF NOT EXISTS public.users
+(
+  updated timestamp with time zone,
+  notifications text[],
+  subscriptions text[],
+  email text NOT NULL,
+  preferences text,
+  password text,
+  token text,
+  watched text[],
+  feed_needs_update boolean,
+  CONSTRAINT users_email_key UNIQUE (email)
+);
+
+GRANT ALL ON TABLE public.users TO current_user;
+
+-- Index: public.email_unique_idx
+
+-- DROP INDEX public.email_unique_idx;
+
+CREATE UNIQUE INDEX IF NOT EXISTS email_unique_idx
+  ON public.users
+  USING btree
+  (lower(email) COLLATE pg_catalog."default");
+