feat: backend-agnostic vector-sync gauges (pending/documents/chunks)

The only queue metric, mcp_vector_sync_queue_size, was updated inline by the
single-user consumer (processor_task) but never by the multi-user consumer
(oauth_processor_task). On multi-user tenants (e.g. blackbox-demo, 5 users) the
gauge read 0 for 24h while the live anyio buffer held ~2214 pending documents
(shown by /api/v1/vector-sync/status). The "indexed" figure was also a chunk
count (16039 points ≈ 480 docs) mislabelled as documents.

Publish a consumer-independent snapshot from a periodic task
(vector/metrics_publisher.vector_sync_metrics_task), spawned in BOTH lifespan
task groups (single-user and multi-user) and every queue backend:
- mcp_vector_sync_pending_documents — outstanding work via
  ingest_status.get_ingest_pending() (anyio buffer depth or procrastinate
  todo+doing); also keeps the legacy queue_size gauge meaningful on all paths.
- mcp_vector_sync_indexed_documents — distinct documents, counted exactly and
  cheaply via the one chunk_index=0 point per document (no facet).
- mcp_vector_sync_indexed_chunks — total non-placeholder points.

The /api/v1/vector-sync/status endpoint now returns indexed_documents (distinct
docs) AND indexed_chunks separately, so documents and chunks are no longer
conflated. The publisher uses approximate Qdrant counts (every-N-seconds gauge);
the on-demand endpoint counts exactly. New knob:
VECTOR_SYNC_METRICS_REFRESH_INTERVAL (default 20s). Fail-safe: a metrics refresh
never disturbs ingest.

BREAKING CHANGE: /api/v1/vector-sync/status field `indexed_documents` now holds
the distinct-document count (was the chunk count); the chunk count moved to the
new `indexed_chunks` field. The Astrolabe UI + the nc_get_vector_sync_status MCP
tool / userinfo page are harmonized in a follow-up (Deck #195).

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

nextcloud_mcp_server/observability/metrics.py

@@ -166,6 +166,30 @@ vector_sync_queue_size = Gauge(
     "Current number of documents in processing queue",
 )
 
+# Outstanding ingest work (queued + in-flight), backend-agnostic. Published by
+# the periodic vector_sync_metrics_task from ingest_status.get_ingest_pending(),
+# so it is correct on every consumer path (single-user processor_task AND
+# multi-user oauth_processor_task) and every queue backend (anyio buffer depth
+# or procrastinate todo+doing) — unlike the per-loop update of
+# ``vector_sync_queue_size``, which only ran on the single-user path.
+vector_sync_pending_documents = Gauge(
+    "mcp_vector_sync_pending_documents",
+    "Outstanding ingest documents (queued or in-flight, not yet processed)",
+)
+
+# Corpus size in the vector store. ``indexed_documents`` counts distinct
+# documents (one chunk_index=0 point per document); ``indexed_chunks`` counts
+# every non-placeholder point. The two differ by the chunk fan-out (~N chunks
+# per document), which is why a single "indexed" figure is ambiguous.
+vector_sync_indexed_documents = Gauge(
+    "mcp_vector_sync_indexed_documents",
+    "Distinct documents indexed in the vector store (non-placeholder)",
+)
+vector_sync_indexed_chunks = Gauge(
+    "mcp_vector_sync_indexed_chunks",
+    "Total indexed chunks (non-placeholder points) in the vector store",
+)
+
 qdrant_operations_total = Counter(
     "mcp_qdrant_operations_total",
     "Total Qdrant vector database operations",
@@ -520,6 +544,21 @@ def update_vector_sync_queue_size(size: int) -> None:
     vector_sync_queue_size.set(size)
 
 
+def update_vector_sync_pending_documents(count: int) -> None:
+    """Set the outstanding-ingest-work gauge (queued + in-flight documents)."""
+    vector_sync_pending_documents.set(count)
+
+
+def update_vector_sync_indexed_documents(count: int) -> None:
+    """Set the distinct-indexed-documents gauge."""
+    vector_sync_indexed_documents.set(count)
+
+
+def update_vector_sync_indexed_chunks(count: int) -> None:
+    """Set the total-indexed-chunks gauge."""
+    vector_sync_indexed_chunks.set(count)
+
+
 def record_document_parse(
     processor: str,
     tier: str,