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/search/algorithms.py

@@ -289,6 +289,8 @@ class SearchAlgorithm(ABC):
         doc_type: str | None = None,
         *,
         accessible_owners: list[str] | None = None,
+        modified_after: int | None = None,
+        modified_before: int | None = None,
         **kwargs: Any,
     ) -> list[SearchResult]:
         """Execute search with the given parameters.
@@ -304,6 +306,12 @@ class SearchAlgorithm(ABC):
                 buried in ``**kwargs`` — so a misspelled keyword is a type error
                 instead of a silent fall back to self-only scope. ``None`` means
                 self-only (``[user_id]``).
+            modified_after: Optional inclusive lower bound on the document's
+                ``modified_at`` payload field (Unix seconds, UTC). Declared
+                explicitly for the same discoverability/type-safety reason as
+                ``accessible_owners`` (ADR-027). ``None`` ⇒ open-ended.
+            modified_before: Optional inclusive upper bound on ``modified_at``
+                (Unix seconds, UTC). ``None`` ⇒ open-ended.
             **kwargs: Algorithm-specific parameters
 
         Returns: