feat(friendships): implement FriendshipService with TDD

- Add send_request(), accept_request(), decline_request(), cancel_request()
- Add get_inbox(), get_outgoing_requests(), get_friends_list(), remove_friend()
- Add unit tests for all service methods (14 tests)
- Update FriendRequestResponse schema to include 'canceled' status
- Follow async SQLAlchemy patterns and BaseService conventions

backend/src/v1/friendships/schemas.py

@@ -24,7 +24,7 @@ class FriendRequestResponse(BaseModel):
     sender: UserBasicInfo
     recipient: UserBasicInfo
     content: str | None
-    status: Literal["pending", "accepted", "rejected"]
+    status: Literal["pending", "accepted", "rejected", "canceled"]
     created_at: datetime