Harden the Redis layer for production.

- Locks no longer fail open: an unreachable backend throws
  LockUnavailableError and withLock skips the run (or opts into running
  unlocked, as template seeding does). The lnbits payment poller keeps
  polling through an outage, made safe by only sending confirmation
  emails when a row actually transitioned.
- Rate limiter INCR+PEXPIRE now runs as one Lua script, self-healing
  counters stranded without a TTL.
- Per-email login lockout moves to a Redis-backed store shared across
  replicas (in-memory fallback preserved), case-insensitive on email.
- Graceful shutdown on SIGTERM/SIGINT: stop periodic jobs, close the
  server, force-close lingering SSE sockets, close Redis.
- Active PING probe backs the health flag; /health reports last ping.
  SSE payment streams also poll the DB as a pub/sub-gap fallback.
- Scale compose gains requirepass, maxmemory 256mb with noeviction, and
  an authenticated healthcheck; .env.example documents passwords, TLS
  (rediss://), and DB-index selection.
- First tests in the repo: vitest + ioredis-mock covering the lock,
  rate limiter, and login lockout stores (27 tests).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/.env.example

@@ -22,10 +22,18 @@ DATABASE_URL=./data/spanglish.db
 # Note: running more than one instance requires DB_TYPE=postgres. SQLite is a
 # single local file and cannot be shared safely across instances.
 
-# Redis connection URL. When set, the cache, rate limiter, pub/sub (real-time
-# payment events), distributed locks, and the email hourly cap are shared across
-# all instances. When unset, each instance uses in-memory equivalents.
+# Redis connection URL. When set, the cache, rate limiter, login lockout,
+# pub/sub (real-time payment events), distributed locks, and the email hourly
+# cap are shared across all instances. When unset, each instance uses in-memory
+# equivalents.
+#
+# In production always set a password (requirepass on the server) and put it in
+# the URL. Use the rediss:// scheme for TLS (handled natively by the client),
+# and an optional /N path to select a DB index when sharing a Redis instance
+# with other applications.
 # REDIS_URL=redis://localhost:6379
+# REDIS_URL=redis://:your-redis-password@redis.internal:6379
+# REDIS_URL=rediss://:your-redis-password@redis.example.com:6380/1
 
 # Optional S3-compatible object storage for media uploads (e.g. Garage, MinIO,
 # AWS S3). When S3_ENDPOINT and S3_BUCKET are set, uploads go to the bucket and