fix(vector): address PR review round 15 — concurrency, pagination, stale coercion

Defer publication of `_qdrant_client` until after the in-lock backfill +
payload-index migration awaits complete. The fast-path check at the top of
`get_qdrant_client` reads the singleton without holding the init lock, so
publishing the constructed-but-unmigrated client let concurrent fast-path
callers fire filtered searches before `_ensure_payload_indexes` ran —
producing HTTP 400 ("Index required but not found") on Qdrant Cloud strict
mode. Local `provisional` is now used for every await inside the lock; the
global is assigned exactly once, last.

Replace the five hand-rolled `scroll(..., limit=10000)` calls in
`vector/scanner.py` (notes / files / news / deck-cards deletion tracking,
plus the timestamp scroll) with a single paginated `_scroll_all_points`
helper. The previous single-page cap silently dropped deletion-tracking
points beyond the first 10 k for any user past that threshold. Pagination
follows Qdrant's documented contract (loop until `next_page_offset is
None`) with a fixed per-page `_DELETION_TRACKING_PAGE_SIZE = 1024`.

Extract `_create_one_payload_index` from `_ensure_payload_indexes` to drop
its cognitive complexity below the SonarQube limit (17 → ≤ 15) without
losing the per-field error-containment rationale; every comment is
preserved verbatim on the helper.

Drop the stale `SearchResult.id` `int | str` comment and the redundant
`str(d)` coercion in `_verify_news_items` — the contract has been
str-only since the producer-side stringification landed earlier in this
PR.

Fix eight `doc_id=<int>` test calls in `test_chunk_context_offset_gate.py`
that violated the `doc_id: str` signature of `get_chunk_with_context`,
plus align `_make_result` in `test_verification.py` to coerce `id=str(...)`
matching the production contract — and update 30+ assertions from int
sets (`{1, 2, 3}`) to str sets (`{"1", "2", "3"}`) so the tests now model
the post-PR `SearchResult.id: str` reality end-to-end. Previously these
were masked by the `str(d)` coercion now removed from production.

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

nextcloud_mcp_server/search/verification.py

@@ -389,9 +389,9 @@ async def _verify_news_items(
     # above for why this is narrower than the API-response failure path.
     accessible: set[str] = set()
     for d in doc_ids:
-        # SearchResult.id is `int | str` (D1: forward-compat widening). Coerce
-        # to str so the validator's regex applies consistently to both shapes.
-        if not is_valid_nextcloud_doc_id(str(d)):
+        # SearchResult.id is always str (Qdrant payload doc_id is keyword-
+        # indexed; producers stringify on write). Pass through verbatim.
+        if not is_valid_nextcloud_doc_id(d):
             logger.warning(
                 "Malformed news_item doc_id %r in verifier; keeping (cannot verify)",
                 d,