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>

Dockerfile

@@ -7,6 +7,7 @@ COPY --from=ghcr.io/astral-sh/uv:0.9.10@sha256:29bd45092ea8902c0bbb7f0a338f0494a
 # 2. sqlite for development with token db
 RUN apt update && apt install --no-install-recommends --no-install-suggests -y \
     git \
+    tesseract-ocr \
     sqlite3 && apt clean
 
 WORKDIR /app
@@ -17,5 +18,6 @@ RUN uv sync --locked --no-dev --no-editable --no-cache
 
 ENV PYTHONUNBUFFERED=1
 ENV VIRTUAL_ENV=/app/.venv
+ENV TESSDATA_PREFIX=/usr/share/tesseract-ocr/5/tessdata
 
 ENTRYPOINT ["/app/.venv/bin/nextcloud-mcp-server", "--host", "0.0.0.0"]