feat(server): make database and OAuth providers optional configuration

All external services (database, Discord/GitHub OAuth, S3) can now be individually disabled by omitting their environment variables. The server gracefully degrades functionality when services are unavailable. Partial configuration of any service group triggers a clear error at startup.

- Database: Falls back to dummy pool when DATABASE_URL is unset
- OAuth: Providers only registered when credentials are complete
- S3: Image storage disabled when credentials are missing
- Health checks reflect actual configuration state

pacman/Justfile

@@ -2,6 +2,10 @@ set shell := ["bash", "-c"]
 
 binary_extension := if os() == "windows" { ".exe" } else { "" }
 
+# Run cargo-vcpkg build for SDL2 dependencies
+vcpkg:
+	cargo vcpkg build
+
 # Run the game, pass args (e.g., `just pacman::run -r` for release)
 run *args:
     cargo run -p pacman {{args}}