Blockchain RPC Router Restoration:
✅ GET /head ENDPOINT: Restored from dummy to real implementation
- router.py: Query actual Block table for chain head instead of returning dummy data
- Added default chain_id from settings when not provided
- Added metrics tracking (total, success, not_found, duration)
- Returns real block data: height, hash, timestamp, tx_count
- Raises 404 when no blocks exist instead of returning zeros
