fix: add metrics-interval validator + type/test gaps (review #850)

- Add Validator("VECTOR_SYNC_METRICS_REFRESH_INTERVAL", gte=1) so a 0/negative
  value can't turn the publish loop into a busy-spin.
- Annotate count_indexed's qdrant_client param as AsyncQdrantClient.
- Add tests: exact kwarg is forwarded to qdrant count, and the placeholder
  filter matches False (excludes placeholders) with chunk_index pinned to 0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/config.py

@@ -271,6 +271,7 @@ _dynaconf = Dynaconf(
         Validator("VECTOR_SYNC_SCAN_INTERVAL", gte=1),
         Validator("VECTOR_SYNC_PROCESSOR_WORKERS", gte=1),
         Validator("VECTOR_SYNC_QUEUE_MAX_SIZE", gte=1),
+        Validator("VECTOR_SYNC_METRICS_REFRESH_INTERVAL", gte=1),
         Validator("VECTOR_SYNC_USER_POLL_INTERVAL", gte=1),
         Validator("VERIFICATION_CONCURRENCY", gte=1),
         Validator("DOCUMENT_CHUNK_SIZE", gte=1),