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.

docker/prod/compose.yml

@@ -60,6 +60,7 @@ services:
       - GITHUB_ID=${GITHUB_ID:?err}
       - GITHUB_SECRET=${GITHUB_SECRET:?err}
       - REDIS_URL=${REDIS_URL:?err}
+      - REDIS_KEY_PREFIX=${REDIS_KEY_PREFIX:-}
       - NEXT_PUBLIC_IS_CLOUD=${NEXT_PUBLIC_IS_CLOUD:-false}
       - API_RATE_LIMIT=${API_RATE_LIMIT:-1}
       - SMTP_HOST=${SMTP_HOST:-smtp.usesend.com}