fix: use docker-compose.override.yml for server-specific config

- Remove :ro and network: host from base docker-compose.yml
- Add docker-compose.override.yml.example with network/healthcheck template
- Add docker-compose.override.yml to .gitignore

This allows git pull without conflicts on the server. Server-specific
settings (external networks, healthchecks) go in the override file.

.gitignore

@@ -53,6 +53,7 @@ Thumbs.db
 
 # ── Deployment ───────────────────────────────────────
 .vercel
+docker-compose.override.yml
 
 # ── User Content (personal gallery config) ───────────
 content/settings.yaml