docs(security): add cross-references between security policy documents - #1159

…ateral of broken main)

Rebases docs/add-security-cross-references onto latest upstream main and fixes
pre-existing breakage that has been failing CI on main itself:

Backend (Backend CI build + Backend npm test + Docker CI):
- schema.prisma: drop duplicated @@unique([transactionHash, eventType]) (Prisma P1012)
- app.ts: serialize BigInt as number in every res.json() (u64 streamId, LabsCrypt#829)
- sse.service.ts: BigInt-safe SSE payload serialization
- soroban-event-worker.ts: fix BigInt/number arithmetic on resume; use workerId in logs
- health.routes.ts: import isRedisAvailable/checkRpcHealth; fix indexerDegraded ref
- soroban-indexer.service.ts: BigInt coercion; drop unused timestampBigInt
- indexer-state.ts: drop createdAt from interface (not in schema)
- admin-rate-limiter.middleware.ts: null-safe x-forwarded-for split
- tests: BigInt fixtures/assertions, vi.hoisted mocks, null-safe index access,
  TestRequest typing, orderBy tiebreaker assertion

Contracts (Soroban CI):
- cargo fmt (rustfmt stable 1.9 drift in stream_contract/src/test.rs)

Frontend (Frontend CI):
- useIncomingStreams.test.tsx: drive withdraw-poll backoff with fake timers
- create-stream-recipient-prefill.test.tsx: raise waitFor timeout (flaky under load)