Refactor app helpers and use AppStateDep in config router

Move service-dependent helper wrappers from app.utils to app.helpers and update config router activation/rollback to use explicit AppState dependency.

backend/app/helpers/log_helpers.py

@@ -0,0 +1,21 @@
+"""Shared log-related helpers for service-layer code."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from app.services.log_service import preview_log as _preview_log
+from app.services.log_service import test_regex as _test_regex
+
+if TYPE_CHECKING:
+    from app.models.config import LogPreviewRequest, LogPreviewResponse, RegexTestRequest, RegexTestResponse
+
+
+def preview_log(req: LogPreviewRequest) -> LogPreviewResponse:
+    """Return log preview results using the shared log service."""
+    return _preview_log(req)
+
+
+def test_regex(req: RegexTestRequest) -> RegexTestResponse:
+    """Validate a regex pattern using the shared log service."""
+    return _test_regex(req)