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/channels.sql

@@ -0,0 +1,25 @@
+-- Table: public.channels
+
+-- DROP TABLE public.channels;
+
+CREATE TABLE IF NOT EXISTS public.channels
+(
+  id text NOT NULL,
+  author text,
+  updated timestamp with time zone,
+  deleted boolean,
+  subscribed timestamp with time zone,
+  CONSTRAINT channels_id_key UNIQUE (id)
+);
+
+GRANT ALL ON TABLE public.channels TO current_user;
+
+-- Index: public.channels_id_idx
+
+-- DROP INDEX public.channels_id_idx;
+
+CREATE INDEX IF NOT EXISTS channels_id_idx
+  ON public.channels
+  USING btree
+  (id COLLATE pg_catalog."default");
+