feat(search): ACL-aware vector filter via Nextcloud Shares lookup

The vector index has always been strictly per-user: every Qdrant payload
carries a `user_id` and the search filter is `user_id == querying_user`.
A file Alice indexed cannot be discovered by Bob even if she has shared
it with him — Bob would have to re-index it under his own user_id to
make it searchable, which means duplicate index entries for every share
recipient.

Switch to ownership-with-ACL-expansion:

- New `nextcloud_mcp_server.search.access_filter` module:
  - `list_accessible_owners(sharing_client, user_id)` calls the OCS
    Sharing API (`shared_with_me=true`) and returns
    `{user_id} ∪ {uid_owner of each share}`. Fails open to `[user_id]`
    so a misbehaving Sharing API doesn't black-hole search.
  - `build_ownership_filter(user_id, accessible_owners)` returns a
    Qdrant `Filter` whose `should` branch matches either the new
    `owner_id IN accessible_owners` field or the legacy `user_id` field.
    The legacy branch keeps points indexed before this change reachable
    without a migration backfill.
- Indexer payload (`vector/processor.py`) now writes `owner_id` alongside
  `user_id`. `DocumentTask` gains an optional `owner_id` field; today the
  scanner always runs as the owner so the processor falls back to
  `user_id`, but the field is plumbed so a future shared-with-me crawler
  can set the true owner without reshaping the payload contract.
- `SemanticSearchAlgorithm.search` and `BM25HybridSearchAlgorithm.search`
  accept `accessible_owners` via kwargs and use the new ownership filter.
  Default behaviour with no kwarg is unchanged (self-only).
- Both user-facing callers — the MCP tool path (`server/semantic.py`) and
  the visualization Starlette route (`auth/viz_routes.py`) — compute
  `accessible_owners` from the authenticated Nextcloud client before
  invoking the search algorithm. Eviction, scanner deletion, placeholder,
  and chunk-context paths intentionally keep the legacy `user_id`
  semantics (those are "operations on a specific user's records", not
  cross-user reads).
- 10 new unit tests in `tests/unit/search/test_access_filter.py` cover
  self-only default, owner expansion, dedup, fallback fields, OCS
  failure, and the legacy `should`-branch shape.

Pairs with cbcoutinho/astrolabe#89 — together they let an Astrolabe user
find content owners have shared with them without going through any
re-authorization flow or re-indexing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/scanner.py

@@ -108,6 +108,12 @@ class DocumentTask:
     metadata: dict[str, int | str] | None = (
         None  # Additional metadata (e.g., board_id/stack_id for deck_card)
     )
+    # UID of the true owner of the indexed object, used by the search-time
+    # ACL filter. None today (scanner always runs as the owner, so the
+    # processor falls back to user_id), but settable so a future
+    # shared-with-me crawl can pass through the actual owner without
+    # reshaping the payload contract.
+    owner_id: str | None = None
 
 
 # Track documents potentially deleted (grace period before actual deletion)