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>

tests/integration/test_acl_shared_search.py

@@ -37,6 +37,7 @@ from nextcloud_mcp_server.search.access_filter import (
     clear_accessible_owners_cache,
     list_accessible_owners,
 )
+from nextcloud_mcp_server.search.context import get_chunk_with_context
 from nextcloud_mcp_server.search.semantic import SemanticSearchAlgorithm
 from nextcloud_mcp_server.search.verification import verify_search_results
 
@@ -149,6 +150,12 @@ async def seeded_semantic(monkeypatch, shared_file):
         "nextcloud_mcp_server.search.semantic.get_embedding_service",
         lambda: provider,
     )
+    # The cached-chunk lookups (get_chunk_with_context) read the client from the
+    # context module — point it at the same in-memory Qdrant.
+    monkeypatch.setattr(
+        "nextcloud_mcp_server.search.context.get_qdrant_client",
+        AsyncMock(return_value=client),
+    )
     yield file_id
     await client.close()
 
@@ -215,3 +222,50 @@ async def test_file_accessible_by_id_resolves_shares(acl_users, shared_file):
     assert await acl_users["bob"].webdav.file_accessible_by_id(fid) is True
     # ...the non-recipient cannot.
     assert await acl_users["diana"].webdav.file_accessible_by_id(fid) is False
+
+
+async def test_cross_user_file_chunk_context(acl_users, seeded_semantic):
+    """End-to-end cross-user FILE chunk context: Bob (a share recipient) gets
+    Alice's cached chunk text, Diana (no share) gets None.
+
+    Exercises the full secure path: the ACL-aware Qdrant cached-chunk lookup
+    (owner_id=alice surfaces for Bob) gated by a real per-file
+    ``file_accessible_by_id`` check against live Nextcloud. Diana fails the gate
+    and is denied even though the chunk is cached. Per-user types are covered by
+    the self-only behaviour elsewhere — this is the file path the feature adds.
+    """
+    file_id = seeded_semantic
+    bob = acl_users["bob"]
+    diana = acl_users["diana"]
+
+    bob_owners = await list_accessible_owners(bob.sharing, "bob")
+    assert "alice" in bob_owners
+
+    ctx = await get_chunk_with_context(
+        nc_client=bob,
+        user_id="bob",
+        doc_id=str(file_id),
+        doc_type="file",
+        chunk_start=0,
+        chunk_end=len(_DOC_TEXT),
+        chunk_index=0,
+        total_chunks=1,
+        accessible_owners=bob_owners,
+    )
+    assert ctx is not None, "Bob (share recipient) must get Alice's cached chunk"
+    assert ctx.chunk_text == _DOC_TEXT
+
+    # Diana has no share → per-file gate denies even though the chunk is cached.
+    diana_owners = await list_accessible_owners(diana.sharing, "diana")
+    denied = await get_chunk_with_context(
+        nc_client=diana,
+        user_id="diana",
+        doc_id=str(file_id),
+        doc_type="file",
+        chunk_start=0,
+        chunk_end=len(_DOC_TEXT),
+        chunk_index=0,
+        total_chunks=1,
+        accessible_owners=diana_owners,
+    )
+    assert denied is None, "Diana (no share) must not get cross-user chunk context"