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>

docs/semantic-search-architecture.md

@@ -5,7 +5,7 @@ This document explains the architecture of the semantic search feature in the Ne
 > [!IMPORTANT]
 > **Status: Experimental**
 > - Disabled by default (`VECTOR_SYNC_ENABLED=false`)
-> - Currently supports **Notes app only** (multi-app architecture ready, additional apps planned)
+> - Currently supports **Notes, Files (PDFs), News items, and Deck cards**
 > - Requires additional infrastructure (Qdrant vector database + Ollama embedding service)
 > - RAG answer generation requires MCP client sampling support
 
@@ -39,9 +39,9 @@ Semantic search enables:
 
 ### Current Support
 
-- **Supported Apps**: Notes (fully implemented)
-- **Planned Apps**: Calendar events, Calendar tasks, Deck cards, Files (with text extraction), Contacts
-- **Architecture**: Multi-app plugin system ready, awaiting implementation
+- **Supported Apps**: Notes, Files (PDFs with text extraction), News items, Deck cards
+- **Planned Apps**: Calendar events, Calendar tasks, Contacts
+- **Architecture**: Multi-app plugin system ready for additional apps
 
 ## System Components