perf(deck): optimize card lookup by storing board_id/stack_id in metadata

Addresses reviewer feedback on PR #395 about O(n²) performance issue.

Changes:
- scanner.py: Add metadata field to DocumentTask with board_id/stack_id
- scanner.py: Populate metadata during deck card scanning (both initial and incremental sync)
- processor.py: Use metadata for O(1) card lookup via get_card() API when available
- processor.py: Fallback to iteration for legacy data without metadata
- context.py: Add _get_deck_metadata_from_qdrant() helper to retrieve metadata from Qdrant
- context.py: Use metadata for fast path lookup in chunk context expansion
- context.py: Add user_id parameter to _fetch_document_text() for metadata retrieval

Performance Impact:
- Before: O(boards × stacks × cards) iteration for each card lookup
- After: O(1) direct API call using stored board_id/stack_id
- Graceful degradation: Falls back to iteration for legacy data

Testing:
- All existing integration tests pass (test_deck_vector_search.py)
- Type checking passes with no new errors

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

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

nextcloud_mcp_server/vector/scanner.py

@@ -36,6 +36,9 @@ class DocumentTask:
     operation: str  # "index" or "delete"
     modified_at: int
     file_path: str | None = None  # File path for files (when doc_id is file_id)
+    metadata: dict[str, int | str] | None = (
+        None  # Additional metadata (e.g., board_id/stack_id for deck_card)
+    )
 
 
 # Track documents potentially deleted (grace period before actual deletion)
@@ -874,6 +877,7 @@ async def scan_deck_cards(
                             doc_type="deck_card",
                             operation="index",
                             modified_at=modified_at,
+                            metadata={"board_id": board.id, "stack_id": stack.id},
                         )
                     )
                     queued += 1
@@ -921,6 +925,7 @@ async def scan_deck_cards(
                                 doc_type="deck_card",
                                 operation="index",
                                 modified_at=modified_at,
+                                metadata={"board_id": board.id, "stack_id": stack.id},
                             )
                         )
                         queued += 1