Add basic backend infrastructure

Signed-off-by: Felipe Cardoso <felipe.cardoso@hotmail.it>

backend/app/models/sample.py

@@ -0,0 +1,11 @@
+from datetime import datetime
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+class Sample(BaseModel):
+    filename: str
+    url: str
+    created_at: datetime
+    step: Optional[int] = None

backend/app/models/training.py

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+from datetime import datetime
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+class TrainingStatus(BaseModel):
+    current_step: int
+    total_steps: int
+    loss: float
+    learning_rate: float
+    eta_seconds: Optional[float]
+    steps_per_second: float
+    updated_at: datetime