feat: add Server-Sent Events endpoint

[code-with-flex]

Description

Implements a Server-Sent Events endpoint at GET /api/events/claims that pushes real-time claim status updates to connected frontend clients. The internal event bus uses Redis pub/sub so every backend instance participates in fan-out, making this horizontally scalable without a centralised broker process. Connections are tracked in a registry that enforces a configurable max connection limit and drains gracefully on SIGTERM.

Related Issue

Closes #202

Changes Made

• events/claim-events.service.ts — Manages dedicated Redis subscriber and publisher connections (separate from the shared RedisService pool, as pub/sub mode blocks a connection). Exposes publish() for the indexer to call after each Prisma transaction commits. Handles malformed messages and Redis unavailability silently (fail open) so SSE outages do not affect indexer operation.
• events/sse-connection.registry.ts — Tracks all active Subject-backed SSE connections. Enforces SSE_MAX_CONNECTIONS (default 500) via a hard limit returning 503. drainAll() completes every subject and is called from ClaimEventsService.onModuleDestroy() to flush in-progress event deliveries before the HTTP server shuts down. Includes inline documentation of backpressure behaviour and max connection guidance.
• events/events.controller.ts — GET /api/events/claims?claimId=<id> using @Sse. Uses OptionalJwtAuthGuard so authenticated wallets get the standard throttle and anonymous clients get the stricter 5 req/min limit. Sets retry: 5000 on every event frame so EventSource clients reconnect within 5 s. Sends 25 s heartbeat comments to survive proxy idle timeouts. No DB transaction is held open during connection lifetime.
• events/events.module.ts — Wires CacheModule, controller, and providers. Exports ClaimEventsService for injection into IndexerModule.
• indexer/indexer.service.ts — Injects ClaimEventsService optionally (@Optional()) and calls publish() after each $transaction block resolves for claim-related topics. The transaction commits first; publish is fire-and-forget (void) so a Redis failure never rolls back a committed indexer write.
• indexer/indexer.module.ts — Adds EventsModule to imports to make ClaimEventsService available.
• app.module.ts — Adds EventsModule to root imports.
• events/__tests__/events.integration.spec.ts — Integration tests covering: registry registration/unregister, broadcast selectivity, connection limit enforcement, drainAll, publish Redis delivery, silent failure on Redis down, message routing, and malformed-message resilience. No real Redis required.

Acceptance Criteria

• Frontend receives a push event within indexer lag + pub/sub latency after a claim status change (typically < 15 s on staging)
• Disconnected clients reconnect via EventSource retry hint and re-subscribe to receive current state
• Load test confirms connection count stays within SSE_MAX_CONNECTIONS; 503 returned when limit is exceeded
• Mocked integration tests pass in CI without real Redis credentials
• SSE connections hold no open DB transactions
• Graceful shutdown drains active connections before HTTP server closes

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