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/views/files.py

@@ -396,9 +396,7 @@ _STEP_TYPE_TO_STAGES: dict[str, list[str]] = {
     "embed_metadata": ["embed_metadata_into_pdf"],
     "compute_embedding": ["compute_embedding"],
     "send_to_destinations": ["finalize_document_storage", "send_to_all_destinations"],
-    # "classify" is defined in PIPELINE_STEP_TYPES but has no Celery log stages yet.
-    # When a classify task is implemented, add its stage key(s) here.
-    "classify": [],
+    "classify": ["classify_document"],
 }
 
 # These internal bookkeeping stages are always shown in the flow regardless of