refactor: add structured_log utility to eliminate logging boilerplate

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/logging_utils.py

@@ -0,0 +1,29 @@
+"""Structured logging utility — eliminates boilerplate across domain services."""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+
+def structured_log(
+    logger: logging.Logger,
+    level: str,
+    event: str,
+    /,
+    **fields: Any,
+) -> None:
+    """Emit a structured log entry.
+
+    The event name is passed as the log message. The JsonLogFormatter in
+    logging_config.py extracts it via record.getMessage() when no explicit
+    'event' key exists in extra — so we do NOT duplicate it.
+
+    Usage:
+        structured_log(logger, "info", "ingestion.run_started", user_id=1, scholar_count=5)
+    """
+    fields.pop("metric_name", None)
+    fields.pop("metric_value", None)
+
+    log_method = getattr(logger, level.lower())
+    log_method(event, extra=fields)