fix: address PR #813 review round 4 (log leak, cross-user chunk ctx, algo, overlap)

1. Don't log unverified result titles: both search algorithms logged top-5
   titles at DEBUG before verify-on-read; with owner-level share expansion the
   unverified set can contain other users' docs. Algorithms now log a count
   only; the verifying callers (server/semantic, viz_routes, api/visualization)
   log verified titles after verify-on-read.

2. Cross-user FILE chunk context: get_chunk_with_context + the Qdrant chunk
   helpers now take accessible_owners and use build_ownership_filter. For files
   the expanded scope is honoured only after a per-file file_accessible_by_id
   check (accessible_owners is owner-level, so the gate prevents a one-file
   share recipient from reading any of the owner's cached chunks). note/deck/
   news stay self-only (per-user APIs) — a documented gap. Both chunk endpoints
   pass accessible_owners.

3. Algorithm usage: SemanticSearchAlgorithm is not dead (it backs the dense-only
   option on the viz/API surfaces); added a clarifying comment in server/
   semantic.py. Additionally wired accessible_owners + verify-on-read into the
   /api/v1 search routes (unified_search, vector_search) so the astrolabe
   surface is ACL-aware too — degrading gracefully to self-only/unverified for
   non-provisioned callers instead of 401.

4. Overlapping conditions: build_ownership_filter no longer lists self in the
   owner_id MatchAny branch (self is already covered by the user_id branch);
   the owner_id branch carries only the OTHER owners.

Tests: build_ownership_filter dedup + chunk-bbox filter-shape updates; new
ACL-aware get_indexed_doc_types, cached-chunk lookup, and end-to-end cross-user
file chunk-context (recipient gets the chunk, non-recipient denied) tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/viz_routes.py

@@ -244,6 +244,16 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
                 verified_results, _dropped = await verify_search_results(
                     nc_client, all_results
                 )
+            # Safe to log titles now: these passed verify-on-read (unverified
+            # titles are never logged — see the search algorithms).
+            if verified_results:
+                logger.debug(
+                    "Top verified results: %s",
+                    ", ".join(
+                        f"{r.doc_type}_{r.id} (score={r.score:.3f}, title='{r.title}')"
+                        for r in verified_results[:5]
+                    ),
+                )
             search_results = verified_results[:limit]
             search_duration = time.perf_counter() - search_start
 
@@ -661,6 +671,10 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
             )
 
         async with nc_client:
+            # Expand to owners who shared content with the caller so the cached
+            # chunk lookup can resolve cross-user SHARED FILES (gated per-file
+            # inside get_chunk_with_context). Same expansion as the search path.
+            accessible_owners = await list_accessible_owners(nc_client.sharing, user_id)
             chunk_context = await get_chunk_with_context(
                 nc_client=nc_client,
                 user_id=user_id,
@@ -671,6 +685,7 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
                 chunk_index=chunk_index,
                 total_chunks=total_chunks,
                 context_chars=context_chars,
+                accessible_owners=accessible_owners,
             )
 
         # Check if context expansion succeeded
@@ -699,12 +714,16 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
         chunk_bbox = None
         page_number = chunk_context.page_number
         if doc_type == "file":
+            # Reaching here means the file chunk context resolved, so access was
+            # already confirmed (get_chunk_with_context gates files by id);
+            # the bbox/page lookup uses the same owner scope for cross-user files.
             qdrant_bbox, qdrant_page = await get_chunk_bbox_and_page_from_qdrant(
                 user_id=user_id,
                 doc_id=doc_id,
                 chunk_index=chunk_index,
                 chunk_start=start,
                 chunk_end=end,
+                accessible_owners=accessible_owners,
             )
             if qdrant_bbox is not None:
                 chunk_bbox = qdrant_bbox