indexer: add a --dry-run mode that validates config and RPC connectivity without writing to the database

[Idaonoli]

Scope

index.ts connects to the configured RPC endpoint and opens the SQLite database as part of normal startup. There's no lightweight way to validate a .env configuration (RPC reachable, at least one contract ID set, DB path writable) before committing to a real indexing run — useful for CI or a new operator's first setup.

Acceptance criteria

• Add a --dry-run (or npm run check-config) mode that validates config and RPC connectivity, prints a summary, and exits without opening/writing the database