feat(search): ADR-027 Phase 1 — modified-date range filter

Add a modified_after/modified_before date-range filter to semantic search,
honoured on both the MCP tool path (BM25HybridSearchAlgorithm) and the
dense-only visualization/API path (SemanticSearchAlgorithm) through one shared
contract.

- Promote modified_after/modified_before to explicit keyword params on the
  SearchAlgorithm ABC and both concrete algorithms; factor the shared
  placeholder+ownership+doc_type+date filter into
  access_filter.build_base_filter_conditions so new filters land in one place.
- nc_semantic_search: accept RFC 3339 / ISO 8601 (or Unix seconds) bounds via
  utils.validation.parse_modified_timestamp; Annotated/Field constraints on the
  numeric args; explicit McpError guard for after > before. Thread the parsed
  bounds through the cross-app and per-doc_type dispatch.
- /api/v1 search endpoints + viz route parse the same formats and 400 on bad or
  inverted ranges.
- Add a modified_at INTEGER payload index to _PAYLOAD_INDEX_FIELDS; the
  idempotent _ensure_payload_indexes() startup path migrates existing
  collections with no content re-index.
- Update ADR-027 to resolve the review feedback (validation placement, shared
  algorithm contract, deferral of nc_semantic_search_answer, payload index,
  RFC-3339-at-the-boundary rationale). Add unit tests.

Refs ADR-027. Deck #177.

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

nextcloud_mcp_server/vector/qdrant_client.py

@@ -52,6 +52,16 @@ _PAYLOAD_INDEX_FIELDS: dict[str, PayloadSchemaType] = {
     "chunk_index": PayloadSchemaType.INTEGER,
     "chunk_start_offset": PayloadSchemaType.INTEGER,
     "chunk_end_offset": PayloadSchemaType.INTEGER,
+    # modified_at is the ADR-027 date-range filter field: searches apply
+    # Range(key="modified_at", gte=..., lte=...) (see
+    # search/access_filter.build_base_filter_conditions). Qdrant requires a
+    # payload index to evaluate a Range efficiently — without one every dated
+    # query full-scans the collection (and 400s on Qdrant Cloud strict mode).
+    # INTEGER (not FLOAT): modified_at is an int Unix-second timestamp on every
+    # point (vector/processor.py, vector/placeholder.py). _ensure_payload_indexes
+    # is idempotent, so existing collections gain this index at startup with no
+    # content re-index and no operator action.
+    "modified_at": PayloadSchemaType.INTEGER,
 }
 
 # Sentinel point that records "this collection has been backfilled to str