fix(quota): prune quota history past a retention window

quota_events had no retention: one row per API spend plus a zero-unit row per live search, kept
forever. The daily history cleanup (audit_gc) now also prunes it past quota_retention_days (a new
Quota setting, default 365 so the admin usage view's longest window stays whole; 0 = forever). The
floor of two days keeps today's rows, which the per-user search cap counts.

backend/app/quota.py

@@ -7,11 +7,13 @@ always be allowed; expensive backfill must yield to the remaining budget.
 
 import contextlib
 import contextvars
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
+from typing import Any, cast
 from zoneinfo import ZoneInfo
 
-from sqlalchemy import func, select
+from sqlalchemy import delete, func, select
 from sqlalchemy.dialects.postgresql import insert as pg_insert
+from sqlalchemy.engine import CursorResult
 from sqlalchemy.orm import Session
 
 from app import sysconfig
@@ -96,6 +98,20 @@ def pacific_today():
     return datetime.now(_PACIFIC).date()
 
 
+def gc(db: Session, retention_days: int) -> int:
+    """Delete quota-history rows older than `retention_days` (<= 0 keeps them forever) and return how
+    many went. Flushes only; the cleanup job commits. Today's rows are never in reach: the per-user
+    search cap counts them, and the floor of two days outlasts any Pacific day (25 h at most)."""
+    if retention_days <= 0:
+        return 0
+    cutoff = datetime.now(timezone.utc) - timedelta(days=max(2, retention_days))
+    # A DELETE answers with a CursorResult; `execute` is typed as the general Result, without rowcount.
+    result = cast(
+        CursorResult[Any], db.execute(delete(QuotaEvent).where(QuotaEvent.created_at < cutoff))
+    )
+    return result.rowcount or 0
+
+
 def pacific_day_start_utc() -> datetime:
     """UTC instant of the current Pacific day's midnight — aligns 'today' with the budget reset."""
     start = datetime.now(_PACIFIC).replace(hour=0, minute=0, second=0, microsecond=0)