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/app.py

@@ -126,6 +126,7 @@ from nextcloud_mcp_server.server import (
 )
 from nextcloud_mcp_server.server.auth_tools import register_auth_tools
 from nextcloud_mcp_server.server.oauth_tools import register_oauth_tools
+from nextcloud_mcp_server.vector.metrics_publisher import vector_sync_metrics_task
 from nextcloud_mcp_server.vector.oauth_sync import (
     oauth_processor_task,
     user_manager_task,
@@ -1744,6 +1745,16 @@ def get_app(transport: str = "streamable-http", enabled_apps: list[str] | None =
                             username,
                         )
 
+                # Publish outstanding-work + corpus gauges on a fixed cadence,
+                # independent of the consumer path and queue backend (fixes the
+                # gauge reading 0 on the multi-user path; see metrics_publisher).
+                await tg.start(
+                    vector_sync_metrics_task,
+                    task_producer,
+                    receive_stream,
+                    shutdown_event,
+                )
+
                 # Expose this long-lived task group to request-path code that
                 # wants to spawn background work (e.g. ADR-019 verify-on-read
                 # eviction). Eviction coroutines have their own try/except, so
@@ -1973,6 +1984,18 @@ def get_app(transport: str = "streamable-http", enabled_apps: list[str] | None =
                                 nextcloud_host_for_sync,
                             )
 
+                    # Publish outstanding-work + corpus gauges on a fixed
+                    # cadence. Critical on this multi-user path: the consumer is
+                    # oauth_processor_task, which never updated the queue gauge,
+                    # so without this the gauge read 0 while the buffer held
+                    # thousands of pending docs (see metrics_publisher).
+                    await tg.start(
+                        vector_sync_metrics_task,
+                        task_producer,
+                        receive_stream,
+                        shutdown_event,
+                    )
+
                     # Expose this long-lived task group to request-path code
                     # that wants to spawn background work (e.g. ADR-019
                     # verify-on-read eviction). Eviction coroutines have their