feat: ownership event, Redis health check, network endpoint, network mismatch guard

[harouns-ux]

Summary

This PR addresses four backend issues in a single branch.

Changes

• No event emitted on successful ownership transfer #1341 — Ownership transfer event: Added backend/src/transfer/ownership-transfer-event.service.ts — an injectable service that emits a typed OwnershipTransferredEvent via EventEmitter2 on every successful transfer.
• Health check reports healthy even when Redis is down #1342 — Redis health check: Updated backend/src/health/health.controller.ts to ping the Redis connection before responding; returns status: degraded with redis: unreachable instead of falsely reporting healthy.
• No endpoint exposes the currently active Stellar network #1343 — Active network endpoint: Added backend/src/network/network.controller.ts — a GET /network endpoint that returns the currently configured STELLAR_NETWORK and STELLAR_HORIZON_URL for operator visibility.
• No guard against verifying a mainnet hash on testnet #1344 — Network mismatch guard: Added backend/src/network/network-mismatch.guard.ts — an injectable guard that throws BadRequestException when a document hash anchored on one network is verified against a different active network.

Closes

Closes #1341
Closes #1342
Closes #1343
Closes #1344

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