fix: add Alembic migration setup to resolve missing script_location error

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/bfb1bb7e-b0da-45b8-b034-b2331488905e

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

backend/app/models/report.py

@@ -28,7 +28,7 @@ class DMARCReport(Base):
     percentage = Column(Integer, nullable=True)
 
     # Processing metadata
-    processed_at = Column(DateTime, default=datetime.utcnow, index=True)
+    processed_at = Column(DateTime, default=datetime.utcnow)
     raw_data = Column(Text, nullable=True)  # Original XML content (optional)
 
     # Relationships