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/auth/viz_routes.py

@@ -39,7 +39,10 @@ from nextcloud_mcp_server.search.context import (
     get_chunk_with_context,
 )
 from nextcloud_mcp_server.search.verification import verify_search_results
-from nextcloud_mcp_server.utils.validation import is_valid_nextcloud_doc_id
+from nextcloud_mcp_server.utils.validation import (
+    is_valid_nextcloud_doc_id,
+    parse_modified_timestamp,
+)
 from nextcloud_mcp_server.vector.oauth_sync import (
     NotProvisionedError,
     get_user_client_basic_auth,
@@ -141,6 +144,31 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
     doc_types_param = request.query_params.get("doc_types", "")
     doc_types = doc_types_param.split(",") if doc_types_param else None
 
+    # Parse ADR-027 modified-date range filter. Accepts RFC 3339 / ISO 8601
+    # datetimes or Unix seconds; normalized to int Unix seconds. Absent ⇒
+    # open-ended. Unparseable input or an inverted range returns 400.
+    try:
+        modified_after = parse_modified_timestamp(
+            request.query_params.get("modified_after"), param_name="modified_after"
+        )
+        modified_before = parse_modified_timestamp(
+            request.query_params.get("modified_before"), param_name="modified_before"
+        )
+    except ValueError as exc:
+        return JSONResponse(
+            {"success": False, "error": str(exc)},
+            status_code=400,
+        )
+    if (
+        modified_after is not None
+        and modified_before is not None
+        and modified_after > modified_before
+    ):
+        return JSONResponse(
+            {"success": False, "error": "modified_after must be <= modified_before"},
+            status_code=400,
+        )
+
     logger.info(
         "Viz search: user=%s, query='%s', algorithm=%s, fusion=%s, limit=%s, doc_types=%s",
         username,
@@ -202,6 +230,8 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
                         doc_type=None,  # Search all types
                         score_threshold=score_threshold,
                         accessible_owners=accessible_owners,
+                        modified_after=modified_after,
+                        modified_before=modified_before,
                     )
                 all_results.extend(unverified_results)
             else:
@@ -222,6 +252,8 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
                             doc_type=doc_type,
                             score_threshold=score_threshold,
                             accessible_owners=accessible_owners,
+                            modified_after=modified_after,
+                            modified_before=modified_before,
                         )
                     all_results.extend(unverified_results)
                 # Sort by score, then cap to the same limit*2 over-fetch budget