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>

tests/unit/vector/test_qdrant_client.py

@@ -73,6 +73,21 @@ def _record(point_id: int | str, doc_id: int | str | None) -> SimpleNamespace:
     return SimpleNamespace(id=point_id, payload=payload)
 
 
+# ---------------------------------------------------------------------------
+# _PAYLOAD_INDEX_FIELDS contract
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.unit
+def test_modified_at_indexed_as_integer():
+    """ADR-027: the date-range filter needs a numeric index on modified_at.
+
+    INTEGER (not FLOAT/DATETIME) because the payload stores an int Unix-second
+    timestamp; a numeric Range filters it without a content re-index.
+    """
+    assert _PAYLOAD_INDEX_FIELDS.get("modified_at") == PayloadSchemaType.INTEGER
+
+
 # ---------------------------------------------------------------------------
 # _ensure_payload_indexes
 # ---------------------------------------------------------------------------