feat: implement AI model API proxy service

backend/app/services/key_selector.py

@@ -0,0 +1,60 @@
+"""Provider key selector with round-robin strategy."""
+import asyncio
+from typing import Optional, Dict
+from ..database import db_connection
+from ..models import ProviderKey
+
+
+class KeySelector:
+    """Round-robin key selector for provider API keys."""
+
+    def __init__(self):
+        self._indices: Dict[int, int] = {}  # provider_id -> current index
+        self._lock = asyncio.Lock()
+
+    async def get_next_key(self, provider_id: int) -> Optional[ProviderKey]:
+        """Get the next available API key for a provider.
+
+        Args:
+            provider_id: The provider ID
+
+        Returns:
+            ProviderKey if available, None if no keys configured
+        """
+        async with self._lock:
+            async with db_connection() as db:
+                cursor = await db.execute(
+                    """SELECT id, provider_id, key, is_active, created_at
+                       FROM provider_keys
+                       WHERE provider_id = ? AND is_active = TRUE""",
+                    (provider_id,)
+                )
+                keys = await cursor.fetchall()
+
+                if not keys:
+                    return None
+
+                # Get or initialize index
+                if provider_id not in self._indices:
+                    self._indices[provider_id] = 0
+
+                # Round-robin selection
+                index = self._indices[provider_id] % len(keys)
+                self._indices[provider_id] = index + 1
+
+                row = keys[index]
+                return ProviderKey(
+                    id=row["id"],
+                    provider_id=row["provider_id"],
+                    key=row["key"],
+                    is_active=row["is_active"],
+                    created_at=row["created_at"]
+                )
+
+    def reset_index(self, provider_id: int):
+        """Reset the round-robin index for a provider."""
+        self._indices.pop(provider_id, None)
+
+
+# Global instance
+key_selector = KeySelector()