feat: deploy CouchDB on hf-pdocker-02 for Obsidian LiveSync (:5984)

Backs the Obsidian 'Second Brain' vault (private repo: git.hobbs.farm/jeremy/
second-brain) with self-hosted multi-device sync. LAN-only for now; no public
Traefik route yet.

- couchdb:3.4 pinned, Watchtower disabled, mem_limit 1g.
- Data on LOCAL disk (/var/lib/couchdb) -- CouchDB file locking breaks over
  NFSv3, same class as guacamole-db and semaphore-db.
- local.ini mounted READ-WRITE, deliberately: CouchDB 3.x writes runtime config
  back into the last-loaded ini, and with :ro it exits 1 and logs NOTHING --
  a silent crash-loop that is very hard to diagnose.
- CORS allows app://obsidian.md and capacitor://localhost (LiveSync desktop +
  mobile origins); a wildcard origin is not allowed alongside credentials.
- require_valid_user everywhere EXCEPT /_up, so the blackbox probe can health
  check it without credentials. Anonymous callers get 401 on all data.
- Admin credentials in Vaultwarden ('CouchDB - Obsidian LiveSync'); .env is
  gitignored, .env.example carries a placeholder.

Prometheus probe added (probe_success=1). Vault database: 'secondbrain'.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

compose/hf-pdocker-02/couchdb/.env.example

@@ -0,0 +1,2 @@
+COUCHDB_USER=obsidian
+COUCHDB_PASSWORD=REPLACE_AT_DEPLOY