feat: add ai cost tracking foundation

ai_knowledge_base_v2/apps/backend/tests/test_ai_cost_tracking.py

@@ -0,0 +1,60 @@
+from __future__ import annotations
+
+from decimal import Decimal
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session
+from sqlalchemy.pool import StaticPool
+
+from app.models import Base
+from app.models.ai_config import ModelConfig
+from app.models.logs import AiRequestLog
+from app.services.ai_request_log_service import AiRequestLogService
+
+
+def _db() -> Session:
+    engine = create_engine("sqlite:///:memory:", connect_args={"check_same_thread": False}, poolclass=StaticPool)
+    Base.metadata.create_all(engine)
+    return Session(engine)
+
+
+def test_ai_request_log_estimates_cost_from_model_price():
+    with _db() as db:
+        model = ModelConfig(
+            id=1,
+            provider="test",
+            api_type="openai_compatible",
+            model_name="test-model",
+            api_url="https://example.com",
+            api_key="secret",
+            input_price_per_1k=Decimal("0.002"),
+            output_price_per_1k=Decimal("0.006"),
+            currency="CNY",
+            timeout_second=30,
+        )
+        db.add(model)
+        db.commit()
+
+        AiRequestLogService.write_success(
+            db,
+            session_id=1,
+            message_id=2,
+            user_id=3,
+            model_id=1,
+            model_name="test-model",
+            prompt="hello",
+            knowledge_ids="1",
+            retrieve_count=2,
+            input_token=1000,
+            output_token=500,
+            cost_ms=120,
+        )
+        db.commit()
+
+        log = db.query(AiRequestLog).one()
+        assert log.estimated_cost == Decimal("0.005000")
+        assert log.currency == "CNY"
+        assert log.knowledge_hit == 1
+        assert log.question_type == "knowledge_grounded"
+        assert "命中知识库" in (log.route_reason or "")
+