Implements four backend realtime/withdrawals issues on test-implement-drips.

[JONAH-6]

Summary

Branch feat/realtime-leaderboard-redis-withdrawal-submit-940-948-949-952 (off
test-implement-drips) resolves all 4 issues:

• Realtime: Emit leaderboard.updated events #952 — Added leaderboard.updated socket event, broadcast to a public leaderboard
  room whenever a confirmed tip changes a creator's rank (wired into the existing
  tip-confirmation flow, best-effort like the existing balance.updated).
• Realtime: Redis adapter for horizontal scaling #948 — Attached the Socket.IO Redis adapter (@socket.io/redis-adapter, using two
  duplicated ioredis connections) so rooms are shared across horizontally scaled
  instances. Gated by a new REALTIME_REDIS_ADAPTER_ENABLED env var (default true;
  disabled in tests).
• Realtime: Per-creator rooms #949 — The per-creator room implementation already existed from earlier work;
  added stronger tests proving targeted delivery (not just "no error thrown").
• Withdrawals: POST /withdrawals/submit (signed tx) #940 — Added POST /withdrawals/submit: broadcasts a wallet-signed transaction via
  Soroban RPC and records a PENDING withdrawal, idempotent by txHash.

Also fixed a pre-existing bug in tips.test.ts (an undefined mockCreateNotification
reference and a mockUserFindUnique typo) that was crashing the entire test file
before any test in it could run — necessary to verify my own change and to unblock
the suite.

Verified: npm run typecheck and npm run lint are clean of anything I touched (only a
pre-existing, unrelated credit.service.ts bug remains). npm run test — my new/fixed
tests all pass (21 new tests + 37 recovered tests); no regressions versus baseline.

PR body to use

Implements four backend realtime/withdrawals issues on test-implement-drips.

What's done

• Leaderboard realtime events: leaderboard.updated is emitted to a public
  leaderboard room whenever a confirmed tip changes a creator's rank. Clients
  subscribe with subscribe:leaderboard / unsubscribe:leaderboard.
• Redis adapter for horizontal scaling: initRealtime attaches
  @socket.io/redis-adapter (two duplicated ioredis connections) so rooms
  are shared across multiple backend instances. Controlled by
  REALTIME_REDIS_ADAPTER_ENABLED (new env var, documented in .env.example
  and src/config/env.ts; defaults to true, disabled in the test suite).
• Per-creator rooms: implementation already existed; added test coverage
  proving tip.created-style events are only delivered to sockets that
  subscribed to that creator's room.
• POST /withdrawals/submit: accepts a wallet-signed transaction XDR,
  broadcasts it via Soroban RPC, and records a PENDING withdrawal row
  (idempotent by txHash). Validated with Zod; errors use AppError.

Also fixed

• tips.test.ts had a pre-existing bug (undefined mockCreateNotification,
  a mockUserFindUnique typo) that crashed the entire file before any test
  could run. Fixed so the suite — and the new leaderboard test in it — can
  actually execute.

Testing

• npm run typecheck — clean (no new errors)
• npm run lint — clean (no new warnings/errors)
• npm run test — all new/fixed tests pass; no regressions

Closes #952
Closes #948
Closes #949
Closes #940

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[Akanimoh12]

Great work