feat: Add metrics instrumentation for queue, health, and database operations

Implement Prometheus metrics to populate empty Grafana dashboard panels.

## Phase 1: Queue Size Metrics ✅
**File**: `processor.py`
- Track vector sync queue depth in real-time
- Update metric after receiving and processing each document
- Update metric during timeout (empty queue)
- Enables: "Processing Queue Depth" panel

## Phase 2: Health Check Metrics ✅
**File**: `app.py`
- Add Nextcloud connectivity check with timing
- Add Qdrant health check with timing
- Record dependency health status (up/down)
- Record health check duration
- Enables: 4 health status panels + health check duration panel

## Phase 3: Database Operation Metrics (Partial) ⏳
**File**: `storage.py`
- Instrument `store_refresh_token()` method
- Track SQLite INSERT operation timing and success/error status
- Enables: Partial data for database operation latency panel

## Metrics Now Exposed

### Queue Metrics:
- `mcp_vector_sync_queue_size` - Real-time queue depth

### Health Metrics:
- `mcp_dependency_health{dependency="nextcloud"}` - UP/DOWN status
- `mcp_dependency_health{dependency="qdrant"}` - UP/DOWN status
- `mcp_dependency_check_duration_seconds{dependency}` - Health check latency

### Database Metrics:
- `mcp_db_operations_total{db="sqlite",operation="insert"}` - Operation count
- `mcp_db_operation_duration_seconds{db="sqlite",operation="insert"}` - Operation latency

## Dashboard Impact

**Panels Now Populated** (7/34 panels):
- ✅ Processing Queue Depth
- ✅ Nextcloud Health
- ✅ Qdrant Health
- ✅ Health Check Duration
- ✅ Database Operation Latency (partial)
- ✅ Vector sync panels (already working from PR #292)

**Panels Still Empty** (remaining work):
- ⏳ OAuth panels (4): Token validations, exchanges, cache hit rate, refresh ops
- ⏳ MCP tool panels (3): Call volume, error rates, execution duration
- ⏳ Database panel: Needs more SQLite operations instrumented (~29 remaining)

## Testing

Verified metric definitions exist and will be recorded on next deployment.

## Next Steps

Phase 4: OAuth token metrics (unified_verifier.py, context_helper.py, storage.py)
Phase 5: MCP tool metrics (all server/*.py files with @mcp.tool())
Phase 3 completion: Remaining 29 database operations in storage.py

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nextcloud_mcp_server/vector/processor.py

@@ -18,6 +18,7 @@ from nextcloud_mcp_server.embedding import get_embedding_service
 from nextcloud_mcp_server.observability.metrics import (
     record_qdrant_operation,
     record_vector_sync_processing,
+    update_vector_sync_queue_size,
 )
 from nextcloud_mcp_server.observability.tracing import trace_operation
 from nextcloud_mcp_server.vector.document_chunker import DocumentChunker
@@ -61,11 +62,21 @@ async def processor_task(
             with anyio.fail_after(1.0):
                 doc_task = await receive_stream.receive()
 
+            # Update queue size metric after receiving
+            stream_stats = receive_stream.statistics()
+            update_vector_sync_queue_size(stream_stats.current_buffer_used)
+
             # Process document
             await process_document(doc_task, nc_client)
 
+            # Update queue size metric after processing
+            stream_stats = receive_stream.statistics()
+            update_vector_sync_queue_size(stream_stats.current_buffer_used)
+
         except TimeoutError:
-            # No documents available, continue
+            # No documents available, update metric to show empty queue
+            stream_stats = receive_stream.statistics()
+            update_vector_sync_queue_size(stream_stats.current_buffer_used)
             continue
 
         except anyio.EndOfStream: