feat: Add context expansion to semantic search with chunk overlap removal

Implements optional context expansion for semantic search results that
fetches adjacent chunks (N-1 and N+1) from Qdrant to provide before/after
context. Removes configurable chunk overlap (default 200 chars) to avoid
duplicate text appearing in both context and excerpt.

Key changes:
- Add include_context and context_chars parameters to nc_semantic_search
  and nc_semantic_search_answer tools
- Implement Qdrant cache fast path for chunk retrieval (avoids re-fetching
  and re-parsing documents, especially important for PDFs)
- Add _get_chunk_by_index_from_qdrant() to fetch adjacent chunks
- Remove chunk overlap from before_context (last N chars) and after_context
  (first N chars) to prevent duplicate text
- Fetch context in parallel with anyio.Semaphore (max 20 concurrent)
- Pass through page_number from SearchResult to SemanticSearchResult
- Remove document-level deduplication (keep chunk-level dedup from algorithm)

Context expansion is opt-in via include_context=true parameter. When enabled:
- Populates has_context_expansion, marked_text, before_context, after_context
- Adds truncation flags when context exceeds context_chars limit
- Falls back to document fetch for legacy data with truncated excerpts

Related: nextcloud_mcp_server/search/context.py:87-382,
         nextcloud_mcp_server/server/semantic.py:161-255

nextcloud_mcp_server/vector/processor.py

@@ -391,7 +391,7 @@ async def _index_document(
                     "doc_type": doc_task.doc_type,
                     "is_placeholder": False,  # Real indexed document (not placeholder)
                     "title": title,
-                    "excerpt": chunk.text[:200],
+                    "excerpt": chunk.text,  # Full chunk text (up to chunk_size, default 2048 chars)
                     "indexed_at": indexed_at,
                     "modified_at": doc_task.modified_at,
                     "etag": etag,