refactor: enhance installation process with support for full-stack and app-only modes, update environment configurations, and improve database migration handling

.env.example

@@ -1,5 +1,8 @@
 # Neta self-host environment
 
+# Use app-only when Neta connects to an external Supabase-compatible backend.
+NETA_INSTALL_MODE=app-only
+
 # Public URL where users open Neta.
 NEXT_PUBLIC_SITE_URL=http://localhost:3000
 
@@ -18,6 +21,10 @@ NEXT_PUBLIC_SUPABASE_ANON_KEY=
 # Keep this secret. It is only used server-side.
 SUPABASE_SERVICE_ROLE_KEY=
 
+# Optional internal Supabase URL for Docker networks.
+# In bundled mode this is set to http://neta-supabase-proxy:8000 by docker-compose.full.yml.
+# SUPABASE_INTERNAL_URL=
+
 # Optional: direct Postgres connection string used only by bash ./scripts/apply-migrations.sh.
 # Do not expose this to browsers. It is not required by the web container.
 # DATABASE_URL=postgresql://postgres:password@host:5432/postgres