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/processor.py

@@ -706,6 +706,16 @@ async def _index_document(
                 },
                 payload={
                     "user_id": doc_task.user_id,
+                    # owner_id is the UID of the file's owner — what
+                    # search-time ACL expansion filters on. Today the scanner
+                    # always runs as the file's owner (per-user crawl, only
+                    # surfaces files the user owns or that fall under their
+                    # WebDAV root), so owner_id == user_id is correct for
+                    # every doc type indexed here. The fields are kept
+                    # separate so a future indexer change that lets a user
+                    # crawl shared-with-them content can set owner_id to the
+                    # true owner without losing the "who indexed this" trail.
+                    "owner_id": doc_task.owner_id or doc_task.user_id,
                     "doc_id": doc_task.doc_id,
                     "doc_type": doc_task.doc_type,
                     "is_placeholder": False,  # Real indexed document (not placeholder)