Fix CI failures: add backend/conftest.py for module resolution and run black formatting

- Add backend/conftest.py that inserts the backend directory into sys.path,
  fixing ModuleNotFoundError when pytest runs from the backend/ directory
  (as CI does with `cd backend && pytest tests/`)
- Run black formatter on all 28 backend files that needed reformatting
- All 53 tests pass with both `pytest tests/` and `python -m pytest tests/`

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/beb47db2-da25-416a-8fb0-c1452a3b22a7

backend/app/models/__init__.py

@@ -1,12 +1,31 @@
 """Models package"""
+
 from app.models.database_models import (
-    User, MailAccount, ProcessingRun, ProcessingLog,
-    NotificationConfig, MailServerPreset, SubscriptionPlan, AuditLog,
-    SubscriptionTier, MailProtocol, AccountStatus, NotificationChannel
+    User,
+    MailAccount,
+    ProcessingRun,
+    ProcessingLog,
+    NotificationConfig,
+    MailServerPreset,
+    SubscriptionPlan,
+    AuditLog,
+    SubscriptionTier,
+    MailProtocol,
+    AccountStatus,
+    NotificationChannel,
 )
 
 __all__ = [
-    "User", "MailAccount", "ProcessingRun", "ProcessingLog",
-    "NotificationConfig", "MailServerPreset", "SubscriptionPlan", "AuditLog",
-    "SubscriptionTier", "MailProtocol", "AccountStatus", "NotificationChannel"
+    "User",
+    "MailAccount",
+    "ProcessingRun",
+    "ProcessingLog",
+    "NotificationConfig",
+    "MailServerPreset",
+    "SubscriptionPlan",
+    "AuditLog",
+    "SubscriptionTier",
+    "MailProtocol",
+    "AccountStatus",
+    "NotificationChannel",
 ]