feat: RoadChain ledger API — blockchain infrastructure endpoints

## RoadChain Ledger API

**Source:** `docs/from-drive/company/The-Roadchain-Ledger-Sheet.md`
See also: `docs/from-drive/ai-papers/Entire-Map.md` (Section 6 — DLT governance)

### Tasks
- [ ] Define RoadChain transaction schema
- [ ] Build signed transaction endpoint
- [ ] Implement ledger audit trail
- [ ] Connect to Cece governance layer
- [ ] Expose via `ledger.blackroad.systems`