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/auth/templates/vector_viz.html

@@ -122,7 +122,8 @@
                         <a :href="getNextcloudUrl(result)" target="_blank" style="font-weight: 500; color: #0066cc; text-decoration: none;">
                             <span x-text="result.title"></span>
                         </a>
-                        <div style="font-size: 14px; color: #666; margin-top: 4px;" x-text="result.excerpt"></div>
+                        <div style="font-size: 14px; color: #666; margin-top: 4px;"
+                             x-text="result.excerpt.length > 200 ? result.excerpt.substring(0, 200) + '...' : result.excerpt"></div>
                         <div style="font-size: 12px; color: #999; margin-top: 4px;">
                             Raw Score: <span x-text="result.original_score.toFixed(3)"></span>
                             (<span x-text="(result.score * 100).toFixed(0)"></span>% relative) |