feat: add REDIS_KEY_PREFIX env var for Redis ACL namespace isolation (#365)

* feat: add REDIS_KEY_PREFIX env var for Redis ACL namespace isolation

Adds optional REDIS_KEY_PREFIX env var that prefixes all Redis keys
(BullMQ queues via `prefix` option, cache/lock/rate-limit keys via
`redisKey()` helper). When unset, behavior is unchanged (BullMQ
defaults to "bull:", cache keys are unprefixed).

This enables self-hosters using Redis ACL multi-tenancy to restrict
useSend to its own key namespace (e.g. `~usesend:*`).

16 files changed across env schema, Redis module, 9 BullMQ queue/worker
files, and 5 direct Redis key operation sites.

* docs: add REDIS_KEY_PREFIX to self-host assets and fix docker run example

Add REDIS_KEY_PREFIX env var to docker/prod/compose.yml, .env.example,
.env.selfhost.example, and self-hosting docs. Fix missing trailing
backslashes in standalone docker run example.

* fix(redis): disable ioredis ready check and BullMQ version check

Redis ACL blocks INFO command (in @dangerous category). ioredis uses
INFO for ready check, BullMQ uses it for version detection. Without
these flags, BullMQ workers fail to initialize and silently stop
processing jobs.

- Add enableReadyCheck: false to ioredis connection
- Add skipVersionCheck: true to all 5 Queue + 5 Worker constructors

* fix(redis): add skipVersionCheck to remaining BullMQ job queues

Add skipVersionCheck: true to Queue and Worker constructors in all 4 job
files (campaign-scheduler, cleanup-email-bodies, usage-job,
webhook-cleanup) to match the pattern already used in service files.
This prevents BullMQ version mismatch errors when using REDIS_KEY_PREFIX
with Redis ACL namespace isolation.

.env.selfhost.example

@@ -36,5 +36,8 @@ DOCKER_OUTPUT=1
 API_RATE_LIMIT=1
 AUTH_EMAIL_RATE_LIMIT=5
 
+# Optional: prefix all Redis keys (useful for shared Redis with ACL isolation)
+# REDIS_KEY_PREFIX=""
+
 # used to send important error notification - optional
 DISCORD_WEBHOOK_URL=""