Implement Phase 3: CLI commands (init, server, snapshot, status)

- Cobra CLI with init (create DB + admin user), server (placeholder),
  snapshot (VACUUM INTO backup), status (health check + counts)
- All commands load config via --config flag
- init prompts for username and password (masked)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/pelletier/go-toml/v2 v2.3.0
 	github.com/spf13/cobra v1.10.2
 	golang.org/x/crypto v0.49.0
+	golang.org/x/term v0.41.0
 	modernc.org/sqlite v1.47.0
 )