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/alembic/env.py

@@ -1,4 +1,5 @@
 """Alembic environment configuration"""
+
 from logging.config import fileConfig
 from sqlalchemy import engine_from_config, pool
 from alembic import context
@@ -49,10 +50,7 @@ def run_migrations_online() -> None:
     )
 
     with connectable.connect() as connection:
-        context.configure(
-            connection=connection,
-            target_metadata=target_metadata
-        )
+        context.configure(connection=connection, target_metadata=target_metadata)
 
         with context.begin_transaction():
             context.run_migrations()