feat: preload DBs by regex via COUCH_PRELOAD_DB_INCLUDE

[PeterBaker0]

Summary

Adds COUCH_PRELOAD_DB_INCLUDE so operators can warm ACL caches at boot by matching DB names from Couch GET /_all_dbs (exact or /regex/flags, same syntax as ACL_DB_INCLUDE) without enumerating every data-* project DB in compose/CDK.

• Union with COUCH_PRELOAD_DBS when both are set
• System DBs (_users, _replicator, _global_changes) are never selected by include patterns
• Results still honour ACL_DB_INCLUDE / ACL_DB_EXCLUDE so preload cannot widen ACL install beyond intended scope
• Empty/unset keeps lazy ensure-on-first-request behaviour
• Readiness gates on the resolved preload set (explicit ∪ include matches)

Docs / tests

• README ops table + USER-GUIDE Databases note
• Unit coverage in test/unit/preload.test.ts

Acceptance

1. Regex set → matching DBs get ensure + follower at boot
2. Non-matching / system DBs skipped
3. Empty/unset → current lazy behaviour
4. USER-GUIDE note + small test