chore: prepare repo for public release — scrub internal infra, rewrite README

- Remove owner-specific / legacy deploy files (home/prod/server compose, deploy/).
  The home compose stays as a local untracked file for the maintainer's own deploy.
- Genericise infra-specific code comments (egress-proxy examples) to neutral wording.
- Replace the hardcoded contact email on the legal pages with the instance operator's
  configured admin email, served via the public /auth/config and shown with a neutral
  fallback — so each self-hosted instance shows its own contact.
- Rewrite README for the current app + a copy-paste self-hosting quick start (prebuilt
  image + first-run wizard) with a build-from-source alternative; tidy .env.example.

backend/app/auth.py

@@ -805,4 +805,6 @@ def auth_config(db: Session = Depends(get_db)) -> dict:
     return {
         "google_enabled": google_enabled(db),
         "allow_registration": sysconfig.get_bool(db, "allow_registration"),
+        # Contact shown on the public legal pages (the operator's admin email), if configured.
+        "operator_contact": operator_contact(),
     }