feat: Implement core application structure, AI extraction, persistence, and Telegram bot modules with updated configuration and dependencies.

app/preprocessing/validators.py

@@ -0,0 +1,13 @@
+"""
+Data validation functions.
+"""
+
+def is_valid_expense(data: dict) -> bool:
+    """
+    Validates if the extracted data for an expense is plausible.
+    Stub function.
+    """
+    # Example validation: amount must be positive
+    if "amount" in data and data["amount"] <= 0:
+        return False
+    return True