feat: add automatic event cleanup based on retention setting

Hourly background task deletes events older than configured
retention period (default 90 days). Reads event_retention_days
from settings table. Includes 2 tests.

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

app/cleanup.py

@@ -0,0 +1,35 @@
+"""Automatic event cleanup based on retention settings."""
+
+import asyncio
+import logging
+
+from database import get_connection
+
+logger = logging.getLogger(__name__)
+
+
+async def cleanup_old_events():
+    """Delete events older than the configured retention period. Runs hourly."""
+    while True:
+        try:
+            conn = get_connection()
+            row = conn.execute(
+                "SELECT value FROM settings WHERE key='event_retention_days'"
+            ).fetchone()
+            days = int(row["value"]) if row else 90
+
+            result = conn.execute(
+                "DELETE FROM events WHERE created_at < datetime('now', ? || ' days')",
+                (f"-{days}",),
+            )
+            conn.commit()
+            if result.rowcount > 0:
+                logger.info(
+                    "Cleaned up %d events older than %d days",
+                    result.rowcount,
+                    days,
+                )
+        except Exception:
+            logger.exception("Cleanup failed")
+
+        await asyncio.sleep(3600)