feat(01-05): introduce celery_app + tasks/document_tasks + session-aware classifier

- Add backend/celery_app.py: Celery("docuvault") with Redis broker, JSON
  serialization, and tasks.document_tasks.* routed to documents queue;
  reads REDIS_URL directly from os.environ (no config import — Pitfall 7)
- Add backend/tasks/__init__.py: empty package marker
- Add backend/tasks/document_tasks.py: sync extract_and_classify Celery task
  that calls asyncio.run(_run()) to retrieve bytes from MinIO, extract text
  via extractor, and classify via classifier; classification failure is non-fatal
- Update backend/services/classifier.py: classify_document and
  suggest_topics_for_document now accept session: AsyncSession as first arg;
  all storage.* calls updated to async session-injection pattern
- Add extract_text_from_bytes helper to services/extractor.py for bytes-based
  extraction (used by Celery worker, which retrieves bytes from MinIO)

backend/celery_app.py

@@ -0,0 +1,35 @@
+"""
+Celery application factory for DocuVault.
+
+Kept deliberately minimal to avoid circular imports (Pitfall 7 from RESEARCH.md):
+  - DO NOT import from config (triggers pydantic-settings env-loading side effects)
+  - DO NOT import from main or any FastAPI router module
+  - Only os + celery imported here
+
+REDIS_URL is read directly from os.environ so that this module can be imported
+safely by the Celery worker process without pulling in the FastAPI application
+machinery.
+"""
+import os
+
+from celery import Celery
+
+celery_app = Celery("docuvault")
+
+# Broker + result backend — read REDIS_URL directly from env (not from config.settings)
+_redis_url = os.environ.get("REDIS_URL", "redis://redis:6379/0")
+celery_app.conf.broker_url = _redis_url
+celery_app.conf.result_backend = _redis_url
+
+# JSON-only serialization (safe default; avoids pickle deserialization risks)
+celery_app.conf.task_serializer = "json"
+celery_app.conf.result_serializer = "json"
+celery_app.conf.accept_content = ["json"]
+
+# Route document tasks to the dedicated `documents` queue
+celery_app.conf.task_routes = {
+    "tasks.document_tasks.*": {"queue": "documents"},
+}
+
+# Autodiscover tasks under the `tasks/` package
+celery_app.autodiscover_tasks(["tasks"], force=True)