feat: dedup shared-file parsing/embedding across users in vector sync

A file shared across many users — directly, or via a group folder shared
to a group — was parsed and embedded once per user. Chunk point IDs are
user-agnostic (uuid5(tenant_id, doc_id=fileid, chunk_index)), but the
per-user freshness gate filtered Qdrant by user_id, so two readers
ping-ponged: each overwrote the other's points and each kept seeing "not
indexed for me", reprocessing every scan. Production telemetry (note
386945, finding #5) measured identical docs re-processed every few hours
at 7-13s each, with PDF parse ~62% of per-doc cost.

Layer 1 — tenant-wide dedup:
- Thread the scanner's tag-REPORT etag into the file DocumentTask and the
  chunk payload; index `etag` as a KEYWORD field.
- vector/sharing_state.find_indexed_content scrolls tenant-wide (no
  user_id filter) for a non-placeholder point matching
  (doc_id, doc_type, etag), gated on embedding_identity in Python so a
  model switch correctly forces a re-embed.
- Scanner skips enqueue and the processor skips fetch/parse/embed when a
  match exists (cross-worker race-guard before WebDAV read). Dedup is
  fail-safe: a Qdrant error degrades to "process normally".

Layer 2 — observed-access ACL (no admin / GroupFolders API needed):
- Each point carries `acl_principals` = the set of user:<uid> whose
  scanner has observed (hence can read) the file. The per-user tag REPORT
  is the access oracle; group membership/GroupFolders enumeration is
  admin-only and unavailable in multi-user modes.
- build_ownership_filter ORs MatchAny(acl_principals, ["user:<me>"]) so a
  deduplicated shared/group-folder point surfaces to every reader;
  verify-on-read (_verify_files) remains the precise ACL gate.
- Deletion/eviction become "release one user": drop the principal and
  delete the points only when the set empties, so one user untagging a
  shared file doesn't evict it for the others. Legacy points without the
  field keep the original per-user delete.

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

nextcloud_mcp_server/vector/scanner.py

@@ -31,6 +31,7 @@ from nextcloud_mcp_server.vector.placeholder import (
 )
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
 from nextcloud_mcp_server.vector.queue.ports import TaskProducer
+from nextcloud_mcp_server.vector.sharing_state import claim_existing_index
 
 logger = logging.getLogger(__name__)
 
@@ -448,6 +449,24 @@ async def scan_user_documents(
                     except (ValueError, KeyError):
                         pass
 
+                # Tenant-wide content dedup (Layer 1 / observed-access ACL): if
+                # this exact file content (fileid + etag) is already indexed under
+                # the current embedding model by ANY user in the tenant, skip
+                # re-parsing/re-embedding and just record that this user can read
+                # it. Eliminates the per-user reprocessing ping-pong that arises
+                # because chunk point IDs are user-agnostic (note 386945 #5).
+                etag = str(file_info.get("etag") or "")
+                if etag and await claim_existing_index(file_id, "file", etag, user_id):
+                    _potentially_deleted.pop((user_id, file_id), None)
+                    logger.debug(
+                        "Dedup: file %s (ID: %s) already indexed in tenant; "
+                        "granted access to %s without reprocessing",
+                        file_path,
+                        file_id,
+                        user_id,
+                    )
+                    continue
+
                 if initial_sync:
                     # Send everything on first sync - write placeholder first
                     await write_placeholder_point(
@@ -455,6 +474,7 @@ async def scan_user_documents(
                         doc_type="file",
                         user_id=user_id,
                         modified_at=modified_at,
+                        etag=etag,
                         file_path=file_path,
                     )
                     await send_stream.send(
@@ -465,6 +485,7 @@ async def scan_user_documents(
                             operation="index",
                             modified_at=modified_at,
                             file_path=file_path,
+                            etag=etag,
                         )
                     )
                     file_queued += 1
@@ -525,6 +546,7 @@ async def scan_user_documents(
                             doc_type="file",
                             user_id=user_id,
                             modified_at=modified_at,
+                            etag=etag,
                             file_path=file_path,
                         )
                         await send_stream.send(
@@ -535,6 +557,7 @@ async def scan_user_documents(
                                 operation="index",
                                 modified_at=modified_at,
                                 file_path=file_path,
+                                etag=etag,
                             )
                         )
                         file_queued += 1