Introduce service/repository dependency protocols and tests

backend/app/repositories/protocols.py

@@ -0,0 +1,47 @@
+"""Repository interface protocols for dependency injection.
+
+Routers and services can depend on these abstractions instead of concrete
+module implementations, making the backend easier to test and extend.
+"""
+
+from __future__ import annotations
+
+from typing import Protocol
+
+import aiosqlite
+
+from app.models.auth import Session
+
+
+class SessionRepository(Protocol):
+    """Protocol for session persistence operations."""
+
+    async def create_session(
+        self,
+        db: aiosqlite.Connection,
+        token: str,
+        created_at: str,
+        expires_at: str,
+    ) -> Session:
+        ...
+
+    async def get_session(
+        self,
+        db: aiosqlite.Connection,
+        token: str,
+    ) -> Session | None:
+        ...
+
+    async def delete_session(
+        self,
+        db: aiosqlite.Connection,
+        token: str,
+    ) -> None:
+        ...
+
+    async def delete_expired_sessions(
+        self,
+        db: aiosqlite.Connection,
+        now_iso: str,
+    ) -> int:
+        ...