fix(vector): normalize doc_id to str + add Qdrant keyword payload indexes

Production was logging two cascading classes of Qdrant errors against the
welcomed-malamute deployment:

1. HTTP 400 — "Bad request: Index required but not found for \"doc_id\" of
   one of the following types: [keyword]". The collection was created via
   create_collection() with no payload indexes, so any FieldCondition
   filter on doc_id failed at the Qdrant layer (placeholder writes/reads,
   eviction, search context lookups).

2. Compounding the missing index, producers wrote a mix of int and str
   doc_ids: webhook_parser stringified node_id, scanner stringified note
   IDs, news IDs, and deck card IDs — but the file scanner passed the
   numeric file_id through unchanged. A keyword index would not have
   covered both kinds even if it had existed.

This change:

- Normalizes doc_id to str at every producer site (scanner.py:459,
  DocumentTask.doc_id, indexed_*_ids reads from Qdrant).
- Tightens str|int annotations to str across placeholder.py,
  eviction.py, search/verification.py, search/context.py,
  SearchResult.id, and the auth/api visualization endpoints.
- Defensive str() coercion on doc_id reads in semantic.py /
  bm25_hybrid.py / vector/visualization.py for the transition window
  before the backfill runs.
- Adds an idempotent startup migration in get_qdrant_client():
  - _ensure_keyword_payload_indexes creates KEYWORD indexes for
    doc_id, user_id, and doc_type (tolerates "already exists" 400s).
  - _backfill_doc_id_to_string scrolls the collection once and rewrites
    int doc_ids to str. Skipped after a quick sample shows no legacy
    int payloads.
- Public API preserved: SemanticSearchResult.id stays int via explicit
  int(r.id) narrowing in server/semantic.py — surfaces a TypeError with
  actionable context if a future doc_type ships non-numeric ids.
- Documents the startup migration in docs/configuration.md.

Tests: 11 new unit tests in tests/unit/vector/test_qdrant_client.py
covering happy path / already-exists / unrelated-400 for the index
helpers, and sample-skip / mixed-batch rewrite / payload=None edge cases
for the backfill. 889 unit tests pass.

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

nextcloud_mcp_server/auth/viz_routes.py

@@ -285,7 +285,11 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
                         vector = point.vector
 
                     if vector is not None and point.payload:
-                        doc_id = point.payload.get("doc_id")
+                        # SearchResult.id is str; coerce payload doc_id to match
+                        # so the tuple lookup below succeeds even on legacy
+                        # int-typed payloads written before normalization.
+                        raw_doc_id = point.payload.get("doc_id")
+                        doc_id = None if raw_doc_id is None else str(raw_doc_id)
                         chunk_start = point.payload.get("chunk_start_offset")
                         chunk_end = point.payload.get("chunk_end_offset")
                         chunk_key = (doc_id, chunk_start, chunk_end)
@@ -555,8 +559,7 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
 
         start = int(start_str)
         end = int(end_str)
-        # Convert doc_id to int (all document types use int IDs)
-        doc_id_int = int(doc_id)
+        # doc_id is keyword-indexed in Qdrant as str — pass through verbatim.
 
         user_id = request.user.display_name
         settings = get_settings()
@@ -580,7 +583,7 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
             chunk_context = await get_chunk_with_context(
                 nc_client=nc_client,
                 user_id=user_id,
-                doc_id=doc_id_int,
+                doc_id=doc_id,
                 doc_type=doc_type,
                 chunk_start=start,
                 chunk_end=end,
@@ -620,7 +623,7 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
                         must=[
                             get_placeholder_filter(),
                             FieldCondition(
-                                key="doc_id", match=MatchValue(value=doc_id_int)
+                                key="doc_id", match=MatchValue(value=doc_id)
                             ),
                             FieldCondition(
                                 key="user_id", match=MatchValue(value=username)