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/api/__init__.py

@@ -11,6 +11,7 @@ from app.api.api_tokens import router as api_tokens_router
 from app.api.azure import router as azure_router
 from app.api.backup import router as backup_router
 from app.api.billing import router as billing_router
+from app.api.classification_rules import router as classification_rules_router
 from app.api.database import router as database_router
 from app.api.diagnostic import router as diagnostic_router
 from app.api.dropbox import router as dropbox_router
@@ -82,3 +83,4 @@ router.include_router(imap_accounts_router)
 router.include_router(integrations_router)
 router.include_router(notifications_router)
 router.include_router(scheduled_jobs_router)
+router.include_router(classification_rules_router)