fix(#13): persist Stellar event listener cursor in Postgres, add contract-events processor with idempotency

[d3vobed]

Summary

Replaces Redis TTL-backed cursor storage (60 s default eviction) with persistent Postgres storage. The cursor is now stored in an event_cursors table keyed by network (testnet/mainnet), surviving restarts and eliminating the silent cursor-loss bug that caused duplicate event processing and missed donations.

Changes

Cursor persistence (core fix)

• Add EventCursor Prisma model — unique on network, persists cursor across restarts
• Replace cacheManager.get/set → prisma.eventCursor.findUnique/upsert in StellarEventService
• Network detection — auto-detects "testnet" / "mainnet" from Horizon URL; overridable via STELLAR_NETWORK env var

Contract-events processor (new)

• Create ContractEventsProcessor — consumes the contract-events Bull queue (previously no processor existed)
• Idempotency guard — skips already-processed (txHash, eventType) pairs via ProcessedEvent table
• Event routing:
  • DonationReceived → updates matching pending donations to CONFIRMED
  • MilestoneReleased → updates matching pending milestones to COMPLETED
  • Unknown event types logged as warnings (no crash)

Redis dependency removed

• CACHE_MANAGER / Cache type removed from StellarEventService — cursor no longer stored in Redis
• @nestjs/cache-manager / @keyv/redis dependencies no longer needed for cursor reading; can be cleaned up separately

Testing

• 9 new tests covering:
  • Cursor loaded from Postgres on bootstrap
  • Bootstrap falls back to "now" when no cursor exists
  • Network detection for testnet, mainnet, and explicit STELLAR_NETWORK
  • Processor idempotency (skips duplicate txHash + eventType)
  • DonationReceived routes to donation confirmation
  • MilestoneReleased routes to milestone completion
  • Unknown event types handled gracefully
• All 53 tests pass (44 existing + 9 new)

Closes #13