fix(config): emit background-ops advisory logs once per process

`_get_background_operations_enabled()` was emitting three advisory log
lines (1 INFO + 2 deprecation WARNINGs) on every call. Because
`get_settings()` is intentionally non-cached and runs on every MCP tool
invocation via `get_client()`, the "Automatically enabled background
operations for semantic search in multi-user mode" INFO line was
firing per-request — 569 entries/hour in one production tenant.

Gate the three log emissions behind a module-level
`_bg_ops_advisories_logged` flag, mirroring the existing
`_warn_missing_secret_once` precedent in
`vector/webhook_receiver.py`. The boolean-derivation path stays
unchanged, so the `Settings` value remains fresh per call.

Extends the autouse `_reload_dynaconf_after_test` fixture to reset the
new flag between tests, and adds two regression tests that call
`get_settings()` five times and assert each advisory fires exactly once.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/conftest.py

@@ -16,6 +16,7 @@ def _reload_dynaconf_after_test():
     validation should call _reload_config() explicitly.
     """
     yield
-    from nextcloud_mcp_server.config import _dynaconf
+    from nextcloud_mcp_server import config as _config
 
-    _dynaconf.reload()
+    _config._dynaconf.reload()
+    _config._bg_ops_advisories_logged = False