feat(phase-4-06): Celery daily audit export task + beat schedule (D-17)

- Create backend/tasks/audit_tasks.py: audit_log_daily_export task
  queries yesterday's AuditLog rows, writes CSV, uploads to MinIO
  audit-logs bucket via put_object_raw(bucket='audit-logs', ...)
- All imports deferred inside _run_daily_export() to prevent circular imports
- celery_app.py: add crontab import, beat entry at midnight UTC,
  tasks.audit_tasks.* routed to documents queue

backend/celery_app.py

@@ -14,6 +14,7 @@ import os
 from datetime import timedelta as _timedelta
 
 from celery import Celery
+from celery.schedules import crontab as _crontab
 
 celery_app = Celery("docuvault")
 
@@ -28,18 +29,26 @@ celery_app.conf.result_serializer = "json"
 celery_app.conf.accept_content = ["json"]
 
 # Route document tasks to the dedicated `documents` queue;
-# email tasks to the `email` queue (Phase 2 — D-03)
+# email tasks to the `email` queue (Phase 2 — D-03);
+# audit tasks to the `documents` queue (D-17 — reuse documents worker)
 celery_app.conf.task_routes = {
     "tasks.document_tasks.*": {"queue": "documents"},
     "tasks.email_tasks.*": {"queue": "email"},
+    "tasks.audit_tasks.*": {"queue": "documents"},
 }
 
-# Celery beat schedule: cleanup_abandoned_uploads runs every 30 minutes (D-06)
+# Celery beat schedule:
+#   cleanup-abandoned-uploads — every 30 minutes (D-06)
+#   audit-log-daily-export    — midnight UTC daily (D-17)
 celery_app.conf.beat_schedule = {
     "cleanup-abandoned-uploads": {
         "task": "tasks.document_tasks.cleanup_abandoned_uploads",
         "schedule": _timedelta(minutes=30),
     },
+    "audit-log-daily-export": {
+        "task": "tasks.audit_tasks.audit_log_daily_export",
+        "schedule": _crontab(hour=0, minute=0),
+    },
 }
 celery_app.conf.timezone = "UTC"
 

backend/tasks/audit_tasks.py

@@ -0,0 +1,92 @@
+"""
+Celery tasks for audit log export in DocuVault.
+
+audit_log_daily_export — scheduled by Celery beat at midnight UTC (D-17).
+
+The task queries the audit_log table for the previous calendar day (UTC),
+writes a CSV using the same field whitelist as _audit_to_dict() in api/audit.py,
+and uploads it to the MinIO audit-logs bucket via put_object_raw().
+
+CSV field whitelist: id, event_type, user_id, actor_id, resource_id,
+ip_address, metadata_, created_at — no filename, extracted_text, or
+document content (ADMIN-06, D-15).
+"""
+import asyncio
+
+from celery_app import celery_app
+
+
+@celery_app.task(name="tasks.audit_tasks.audit_log_daily_export")
+def audit_log_daily_export() -> dict:
+    """Synchronous Celery entry-point — delegates to async _run_daily_export via asyncio.run."""
+    return asyncio.run(_run_daily_export())
+
+
+async def _run_daily_export() -> dict:
+    """Async body of audit_log_daily_export.
+
+    Queries yesterday's audit_log rows, writes CSV, and uploads to MinIO
+    audit-logs bucket. All imports are deferred inside this function body to
+    avoid circular imports (same pattern as document_tasks._run).
+    """
+    from datetime import date, datetime, timedelta, timezone
+    import csv
+    import io
+    from db.session import AsyncSessionLocal
+    from db.models import AuditLog
+    from sqlalchemy import select
+    from storage import get_storage_backend
+
+    yesterday = date.today() - timedelta(days=1)
+    start = datetime(yesterday.year, yesterday.month, yesterday.day, tzinfo=timezone.utc)
+    end = start + timedelta(days=1)
+
+    try:
+        async with AsyncSessionLocal() as session:
+            result = await session.execute(
+                select(AuditLog)
+                .where(AuditLog.created_at >= start, AuditLog.created_at < end)
+                .order_by(AuditLog.created_at)
+            )
+            rows = result.scalars().all()
+
+        fields = [
+            "id",
+            "event_type",
+            "user_id",
+            "actor_id",
+            "resource_id",
+            "ip_address",
+            "metadata_",
+            "created_at",
+        ]
+        output = io.StringIO()
+        writer = csv.DictWriter(output, fieldnames=fields)
+        writer.writeheader()
+        for row in rows:
+            writer.writerow({
+                "id": row.id,
+                "event_type": row.event_type,
+                "user_id": str(row.user_id) if row.user_id else None,
+                "actor_id": str(row.actor_id) if row.actor_id else None,
+                "resource_id": str(row.resource_id) if row.resource_id else None,
+                "ip_address": str(row.ip_address) if row.ip_address else None,
+                "metadata_": row.metadata_,
+                "created_at": row.created_at.isoformat(),
+            })
+
+        csv_bytes = output.getvalue().encode("utf-8")
+        key = f"audit-logs/{yesterday.isoformat()}.csv"
+        await get_storage_backend().put_object_raw(
+            bucket="audit-logs",
+            key=key,
+            data=io.BytesIO(csv_bytes),
+            length=len(csv_bytes),
+            content_type="text/csv",
+        )
+        return {"exported": len(rows), "key": key, "date": yesterday.isoformat()}
+
+    except Exception as e:
+        import logging
+        logging.getLogger(__name__).error("audit export failed: %s", e)
+        return {"exported": 0, "error": str(e)}