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/vector/processor.py

@@ -23,6 +23,7 @@ from nextcloud_mcp_server.observability.metrics import (
 )
 from nextcloud_mcp_server.observability.tracing import trace_operation
 from nextcloud_mcp_server.vector.document_chunker import DocumentChunker
+from nextcloud_mcp_server.vector.placeholder import delete_placeholder_point
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
 from nextcloud_mcp_server.vector.scanner import DocumentTask
 
@@ -418,6 +419,20 @@ async def _index_document(
             )
         )
 
+    # Delete placeholder before writing real vectors
+    # This prevents duplicates and cleans up the placeholder state
+    try:
+        await delete_placeholder_point(
+            doc_id=doc_task.doc_id,
+            doc_type=doc_task.doc_type,
+            user_id=doc_task.user_id,
+        )
+    except Exception as e:
+        # Log but don't fail indexing if placeholder deletion fails
+        logger.warning(
+            f"Failed to delete placeholder for {doc_task.doc_type}_{doc_task.doc_id}: {e}"
+        )
+
     # Upsert to Qdrant
     await qdrant_client.upsert(
         collection_name=settings.get_collection_name(),