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>

docs/configuration.md

@@ -329,6 +329,22 @@ OLLAMA_EMBEDDING_MODEL=all-minilm
 - **Switching models requires re-embedding** all documents (may take time for large note collections)
 - **Old collection remains** in Qdrant and can be deleted manually if no longer needed
 
+#### Startup migrations on existing collections
+
+On the first call to `get_qdrant_client()` against an existing collection, the
+server runs two idempotent migrations:
+
+1. **Payload-index creation** — adds `KEYWORD` payload indexes for `doc_id`,
+   `user_id`, and `doc_type`. Required by Qdrant for any `FieldCondition`
+   filter. Cheap; runs even on healthy collections.
+2. **`doc_id` backfill** — rewrites any legacy integer `doc_id` payloads to
+   strings so they match the keyword index. Skipped after a quick sample
+   shows the collection is already clean. On a dirty collection, the full
+   scroll runs once and writes are emitted point-by-point — expect a delay
+   proportional to point count on the first startup after the upgrade.
+
+Both steps emit INFO-level log lines so operators can track progress.
+
 #### Explicit Override
 
 Set `QDRANT_COLLECTION` to use a specific collection name: