refactor: pure env_file stack.env convention across all stacks

Every service now reads stack.env via env_file with container-exact var
names — no ${VAR} parse-time interpolation, no Portainer UI env vars
(except stacks/dev, kept on UI-env interpolation as documented exception).

- media: drop PUID/PGID/TZ env lines (env_file covers); resolve ${ROOT}
- foundry: secrets via env_file; FOUNDRY_PATCH_URLS -> CONTAINER_PATCH_URLS
- monitoring: HOMARR_SECRET_ENCRYPTION_KEY -> SECRET_ENCRYPTION_KEY
- notify: FRESHRSS_USER composed in stack.env; TZ via env_file
- owncloud: container-exact OWNCLOUD_*/MYSQL_* keys; runtime $$ healthcheck
- resume/famapp: composites (DATABASE_URL etc.) written out in stack.env
- authentik: container-exact keys; image tag pinned in compose
- notes: couchdb creds via env_file
- remote: gains stack.env (no secrets; convention completeness)
- CLAUDE.md: env convention, valhalla-lab registration-only note, correct
  on-host Portainer working-copy path (/config/portainer/compose/<id>)

Validated with docker compose config on valhalla for all 15 stacks.

stacks/monitoring/docker-compose.yml

@@ -21,8 +21,6 @@ services:
     networks: [edge]
     env_file:
       - stack.env
-    environment:
-      - SECRET_ENCRYPTION_KEY=${HOMARR_SECRET_ENCRYPTION_KEY}
     ports:
       - "7575:7575"
     volumes:

stacks/monitoring/stack.env

@@ -1,3 +1,3 @@
-# monitoring stack secrets
+# monitoring stack secrets — injected via env_file; container-exact var names.
 # Homarr config encryption key. Generate with: openssl rand -hex 32
-HOMARR_SECRET_ENCRYPTION_KEY=98989838bf06c9e42d77944ce848963a22ab316b55a09b0d199652ef0934394f
+SECRET_ENCRYPTION_KEY=98989838bf06c9e42d77944ce848963a22ab316b55a09b0d199652ef0934394f