feat: Implement Qdrant placeholder state management

Introduces a placeholder-based state tracking system to prevent duplicate
document processing during the gap between scanner queuing and processor
completion.

**Key Changes:**

1. **Placeholder Helper Functions** (`vector/placeholder.py`):
   - `write_placeholder_point()` - Creates zero-vector placeholder when queuing
   - `query_document_metadata()` - Queries for existing entry (placeholder or real)
   - `delete_placeholder_point()` - Removes placeholder before writing real vectors
   - `get_placeholder_filter()` - Filters placeholders from user-facing queries

2. **Scanner Updates** (`vector/scanner.py`):
   - Replace `indexed_at` comparison with `modified_at` comparison
   - Write placeholder before queuing each document
   - Query per-document metadata instead of bulk-querying indexed_at
   - Fixes bug where files were resubmitted every scan cycle

3. **Processor Updates** (`vector/processor.py`):
   - Delete placeholder before upserting real vectors
   - Ensures no duplicate points in Qdrant

4. **Query Filters** (all search files):
   - Add `get_placeholder_filter()` to all user-facing queries
   - Ensures placeholders never appear in search results or visualizations
   - Applied to: bm25_hybrid.py, semantic.py, viz_routes.py, algorithms.py

**Architecture:**
- Placeholders use zero vectors with dimension from embedding service
- Payload includes `is_placeholder: True` flag for filtering
- Status field tracks: "pending", "processing", "completed", "failed"
- Deterministic UUIDs using uuid5 for consistent point IDs

**Impact:**
- Eliminates duplicate processing of same documents
- Fixes race condition where long-running documents get queued multiple times
- Prevents scanner from resubmitting files every scan cycle
- Maintains clean separation between in-flight and indexed documents

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

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

nextcloud_mcp_server/search/algorithms.py

@@ -83,6 +83,7 @@ async def get_indexed_doc_types(user_id: str) -> set[str]:
     from qdrant_client.models import FieldCondition, Filter, MatchValue
 
     from nextcloud_mcp_server.config import get_settings
+    from nextcloud_mcp_server.vector.placeholder import get_placeholder_filter
     from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
 
     logger = logging.getLogger(__name__)
@@ -97,7 +98,10 @@ async def get_indexed_doc_types(user_id: str) -> set[str]:
         scroll_results, _next_offset = await qdrant_client.scroll(
             collection_name=collection,
             scroll_filter=Filter(
-                must=[FieldCondition(key="user_id", match=MatchValue(value=user_id))]
+                must=[
+                    get_placeholder_filter(),  # Exclude placeholders from doc_type discovery
+                    FieldCondition(key="user_id", match=MatchValue(value=user_id)),
+                ]
             ),
             limit=1000,  # Sample size to discover types
             with_payload=["doc_type"],

nextcloud_mcp_server/search/bm25_hybrid.py

@@ -10,6 +10,7 @@ from nextcloud_mcp_server.config import get_settings
 from nextcloud_mcp_server.embedding import get_bm25_service, get_embedding_service
 from nextcloud_mcp_server.observability.metrics import record_qdrant_operation
 from nextcloud_mcp_server.search.algorithms import SearchAlgorithm, SearchResult
+from nextcloud_mcp_server.vector.placeholder import get_placeholder_filter
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
 
 logger = logging.getLogger(__name__)
@@ -112,10 +113,11 @@ class BM25HybridSearchAlgorithm(SearchAlgorithm):
 
         # Build Qdrant filter
         filter_conditions = [
+            get_placeholder_filter(),  # Always exclude placeholders from user-facing queries
             FieldCondition(
                 key="user_id",
                 match=MatchValue(value=user_id),
-            )
+            ),
         ]
 
         # Add doc_type filter if specified

nextcloud_mcp_server/search/semantic.py

@@ -9,6 +9,7 @@ from nextcloud_mcp_server.config import get_settings
 from nextcloud_mcp_server.embedding import get_embedding_service
 from nextcloud_mcp_server.observability.metrics import record_qdrant_operation
 from nextcloud_mcp_server.search.algorithms import SearchAlgorithm, SearchResult
+from nextcloud_mcp_server.vector.placeholder import get_placeholder_filter
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
 
 logger = logging.getLogger(__name__)
@@ -83,10 +84,11 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
 
         # Build Qdrant filter
         filter_conditions = [
+            get_placeholder_filter(),  # Always exclude placeholders from user-facing queries
             FieldCondition(
                 key="user_id",
                 match=MatchValue(value=user_id),
-            )
+            ),
         ]
 
         # Add doc_type filter if specified