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/api/visualization.py

@@ -511,8 +511,8 @@ async def get_chunk_context(request: Request) -> JSONResponse:
                 raise ValueError("end must be greater than start")
         except ValueError as e:
             return JSONResponse({"success": False, "error": str(e)}, status_code=400)
-        # Convert doc_id to int if possible (most IDs are int)
-        doc_id_val: str | int = int(doc_id) if doc_id.isdigit() else doc_id
+        # doc_id is keyword-indexed in Qdrant as str — pass through verbatim
+        # (no int coercion; producers always stringify on write).
 
         # Get Nextcloud host from OAuth context
         oauth_ctx = request.app.state.oauth_context
@@ -537,7 +537,7 @@ async def get_chunk_context(request: Request) -> JSONResponse:
             chunk_context = await get_chunk_with_context(
                 nc_client=nc_client,
                 user_id=user_id,
-                doc_id=doc_id_val,
+                doc_id=doc_id,
                 doc_type=doc_type,
                 chunk_start=start,
                 chunk_end=end,
@@ -570,7 +570,7 @@ async def get_chunk_context(request: Request) -> JSONResponse:
                         must=[
                             get_placeholder_filter(),
                             FieldCondition(
-                                key="doc_id", match=MatchValue(value=doc_id_val)
+                                key="doc_id", match=MatchValue(value=doc_id)
                             ),
                             FieldCondition(
                                 key="user_id", match=MatchValue(value=user_id)