Add Redis distributed lock so sweep workers run on a single active instance

[defimomof2]

• DistributedLockService (src/common/redis/distributed-lock.service.ts) — Redis SET key token NX PX ttl lease + compare-and-delete release (Lua). Wired into GigExpiryWorkerService and RepinWorkerService: each tick acquires a fresh lock (TTL = 1.5x the sweep interval) before running runOnce(), skips the tick if another instance holds it, and releases explicitly on onModuleDestroy. If REDIS_CLIENT is null (REDIS_URL unset), locking fails open — every instance runs independently with a warning logged, preserving today's single-instance behavior instead of silently disabling the sweep; this is documented in the service's doc comment. Existing worker specs updated to inject a fake lock and still pass; new distributed-lock.service.spec.ts covers concurrent-acquire racing (exactly one winner), release/re-acquire, compare-and-delete (can't release a lock you don't hold), and the fail-open path. Closes Add distributed lock so background sweep workers run as a single active instance #182
• Design a Horizontally-Scalable WebSocket Gateway with Redis Fan-out #165 — Horizontally-Scalable WebSocket Gateway with Redis Fan-out — to be done