first commit

backend/app/services/llm/base.py

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+"""Base LLM provider - abstract interface for vision extraction."""
+
+from abc import ABC, abstractmethod
+
+from app.schemas.transaction import TransactionExtractItem
+
+
+class BaseLLMProvider(ABC):
+    """Abstract base for LLM vision providers. Each provider implements extract_from_image."""
+
+    @abstractmethod
+    async def extract_from_image(self, image_bytes: bytes) -> list[TransactionExtractItem]:
+        """
+        Analyze a billing screenshot and return structured transaction list.
+        :param image_bytes: Raw image file content (PNG/JPEG)
+        :return: List of extracted transactions (may be empty or partial on failure)
+        """
+        pass