feat: add friendly error messages, MailDebugRecorder, clear-error endpoint, debug_logging column

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/5d2918de-630e-4a66-8355-1b036c620b1c

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

backend/app/models/database_models.py

@@ -171,6 +171,9 @@ class MailAccount(Base):
     provider_name = Column(String(100), nullable=True)  # e.g., "Gmail", "GMX"
     auto_detected = Column(Boolean, default=False)
 
+    # Debug logging
+    debug_logging = Column(Boolean, default=False)
+
     # Statistics
     total_emails_processed = Column(Integer, default=0)
     total_emails_failed = Column(Integer, default=0)