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>

tests/conftest.py

@@ -61,6 +61,7 @@ from app.main import app as fastapi_app  # noqa: E402
 # Import models to register them with SQLAlchemy Base
 from app.models import (  # noqa: F401, E402
     ApiToken,
+    ClassificationRuleModel,
     DocumentMetadata,
     FileRecord,
     Pipeline,