feat(classify): add rule-based document classification engine, task, and API

Implements the classify pipeline step with:
- Classification rules engine (app/utils/classification_rules.py) with
  pre-built categories (invoice, contract, receipt, letter, report,
  bank_statement, tax_document, insurance, payslip) and support for
  filename patterns, content keywords, and metadata matching rules
- Celery task (app/tasks/classify_document.py) that runs as a pipeline step
- CRUD API (app/api/classification_rules.py) for managing custom rules
- ClassificationRuleModel in app/models.py with migration 027
- Updated pipeline step config_schema and stage mapping
- Comprehensive tests for engine, API, and task

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/celery_worker.py

@@ -22,6 +22,7 @@ from app.tasks.batch_tasks import (  # noqa: F401
     sync_search_index,
 )
 from app.tasks.check_credentials import check_credentials
+from app.tasks.classify_document import classify_document_task  # noqa: F401
 from app.tasks.compute_embedding import backfill_missing_embeddings, compute_document_embedding  # noqa: F401
 from app.tasks.convert_to_pdf import convert_to_pdf  # noqa: F401
 from app.tasks.convert_to_pdfa import convert_to_pdfa  # noqa: F401