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"