Add Docker build & GHCR publish stage, fix CI workflows, update AGENTS.md and testing docs

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/9d256101-34b6-4861-a8cf-7f86f32b54d5

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/models/user.py

@@ -1,8 +1,7 @@
+from app.core.database import Base
 from sqlalchemy import Boolean, Column, Integer, String
 from sqlalchemy.orm import relationship
 
-from app.core.database import Base
-
 
 class User(Base):
     """User model"""