fix(monitoring): exporter auth and reload fixes

- postgres: use lib/pq conninfo DATA_SOURCE_NAME; the postgresql:// URL
  form broke on special chars in the password (net/url: invalid userinfo)
- nginx: flush handlers after deploying stub_status so nginx reloads
  before the exporter starts scraping
- valkey: drop invalid +arcount command from the exporter ACL set
  (Valkey aborted startup parsing the ACL file)

roles/postgres/README.md

@@ -92,7 +92,7 @@ via `postgres_exporter`. Enable it:
 
 ```yaml
 postgres_exporter_enabled: true
-postgres_exporter_password: "<min 12 chars>"  # keep URL-safe
+postgres_exporter_password: "<min 12 chars>"  # avoid spaces/single quotes
 ```
 
 The role creates a `postgres_exporter` monitoring role with the built-in