feat(vector): add Deck card vector search with visualization support

Adds comprehensive vector search support for Nextcloud Deck cards,
including semantic search indexing, chunk preview in the vector viz UI,
and proper deep linking to cards.

**Vector Search Indexing**
- Add deck_card scanning in scanner.py (scan_deck_cards function)
- Index cards from non-archived, non-deleted boards
- Store metadata: board_id, board_title, stack_id, stack_title, card_type, duedate, owner
- Content structure: title + "\n\n" + description (matches indexing format)
- Incremental sync based on lastModified timestamp
- Deletion tracking with grace period

**Vector Visualization Support**
- Add deck_card handler in context.py for chunk preview expansion
- Include board_id in search result metadata (bm25_hybrid.py, semantic.py)
- Expose metadata in viz_routes.py JSON responses
- Update vector-viz.js to construct proper Deck URLs: /apps/deck/board/{board_id}/card/{card_id}
- Update vector_viz.html filter label from "Deck" to "Deck Cards"

**Bug Fixes**
- Skip soft-deleted boards (deletedAt > 0) to prevent 403 Forbidden errors
- Applies to scanner, processor, and context expansion code paths
- Deck API returns deleted boards but rejects stack access with 403

**Testing**
- Add integration tests in test_deck_vector_search.py:
  - test_deck_card_semantic_search: Filtered search with doc_type="deck_card"
  - test_deck_card_appears_in_cross_app_search: Cross-app search includes deck cards
  - test_deck_card_chunk_context: Chunk context fetching for viz preview

**Documentation**
- Update README.md: Add Deck cards to semantic search feature list
- Update semantic-search-architecture.md: Document deck_card support
- Update nc_semantic_search tool documentation

**Type Safety**
- Fix type narrowing for page_boundaries (could be None) using cast()
- Fix scanner.py payload None check for type safety

Resolves vector search for Deck cards across indexing, search, and visualization.

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nextcloud_mcp_server/server/semantic.py

@@ -65,13 +65,13 @@ def configure_semantic_tools(mcp: FastMCP):
         database for optimal relevance. This provides the best of both semantic
         understanding and keyword precision.
 
-        Requires VECTOR_SYNC_ENABLED=true. Currently only "note" documents are
-        fully supported for indexing.
+        Requires VECTOR_SYNC_ENABLED=true. Supports indexing of notes, files,
+        news items, and deck cards.
 
         Args:
             query: Natural language or keyword search query
             limit: Maximum number of results to return (default: 10)
-            doc_types: Document types to search (e.g., ["note", "file"]). None = search all indexed types (default)
+            doc_types: Document types to search (e.g., ["note", "file", "deck_card", "news_item"]). None = search all indexed types (default)
             score_threshold: Minimum fusion score (0-1, default: 0.0)
             fusion: Fusion algorithm: "rrf" (Reciprocal Rank Fusion, default) or "dbsf" (Distribution-Based Score Fusion)
                    RRF: Good general-purpose fusion using reciprocal ranks