feat: Add multi-document Protocol with cross-app search support

Implements NextcloudClientProtocol for multi-document type search following
user requirement that document types are not 1:1 with apps (e.g., Notes app
specializes in markdown, while Files/WebDAV handles multiple file types).

Key Changes:
- NextcloudClientProtocol: Generic protocol with app-specific client properties
- get_indexed_doc_types(): Query Qdrant for actually-indexed document types
- Document dispatch: All algorithms check Qdrant before attempting access
- Cross-type deduplication: Use (doc_id, doc_type) tuples in hybrid RRF

Search Algorithm Updates:
- Semantic: Added _verify_document_access() with dispatch to appropriate client
  - Deduplication by (doc_id, doc_type) tuple
  - Only "note" verification implemented, others return None with info log
- Keyword: Added _fetch_documents() dispatch method
  - Queries Qdrant for available types before fetching
  - Supports cross-app search when doc_type=None
- Fuzzy: Same pattern as keyword search
- Hybrid: Already uses (doc_id, doc_type) for deduplication (no changes needed)

Future-Proof Design:
- File/calendar verification stubs in place
- Clear logging when unsupported types found
- Easy to extend when processor indexes new document types

Currently Supported:
- "note" documents fully implemented and tested
- Other types gracefully handled (logged but skipped)

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

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

nextcloud_mcp_server/search/__init__.py

@@ -10,15 +10,22 @@ All algorithms share the same interface and can be used interchangeably by both
 MCP tools and the visualization pane.
 """
 
-from nextcloud_mcp_server.search.algorithms import SearchAlgorithm, SearchResult
+from nextcloud_mcp_server.search.algorithms import (
+    NextcloudClientProtocol,
+    SearchAlgorithm,
+    SearchResult,
+    get_indexed_doc_types,
+)
 from nextcloud_mcp_server.search.fuzzy import FuzzySearchAlgorithm
 from nextcloud_mcp_server.search.hybrid import HybridSearchAlgorithm
 from nextcloud_mcp_server.search.keyword import KeywordSearchAlgorithm
 from nextcloud_mcp_server.search.semantic import SemanticSearchAlgorithm
 
 __all__ = [
+    "NextcloudClientProtocol",
     "SearchAlgorithm",
     "SearchResult",
+    "get_indexed_doc_types",
     "SemanticSearchAlgorithm",
     "KeywordSearchAlgorithm",
     "FuzzySearchAlgorithm",