feat: TDD Phase 1 — 44 backend API tests passing

Test infrastructure:
- conftest.py with SQLite in-memory DB, isolated per-test cleanup
- pytest.ini with asyncio auto mode
- Fixtures: admin/dba/developer/guest/disabled users, tokens, test instance

Test coverage (44 tests):
- test_auth: login/logout/me/token expiry (8 tests)
- test_users: CRUD + role permissions (8 tests)
- test_instances: CRUD + connection test mock (8 tests)
- test_tickets: CRUD + review/execute workflow + pagination (11 tests)
- test_query: select/drop/delete rejection + guest deny (4 tests)
- test_dashboard: stats + auth check (2 tests)

Backend fix:
- query endpoint now catches ValueError from QueryService → 400 instead of 500

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_auth.py

@@ -0,0 +1,77 @@
+import pytest
+from httpx import AsyncClient
+
+from app.core.security import create_access_token
+
+
+class TestLogin:
+    async def test_login_success(self, client: AsyncClient, admin_user):
+        resp = await client.post("/api/v1/auth/login", json={
+            "username": "admin",
+            "password": "123456",
+        })
+        assert resp.status_code == 200
+        data = resp.json()
+        assert "access_token" in data
+        assert data["token_type"] == "bearer"
+        assert data["user"]["username"] == "admin"
+        assert data["user"]["role"] == "admin"
+
+    async def test_login_wrong_password(self, client: AsyncClient, admin_user):
+        resp = await client.post("/api/v1/auth/login", json={
+            "username": "admin",
+            "password": "wrong",
+        })
+        assert resp.status_code == 401
+
+    async def test_login_nonexistent_user(self, client: AsyncClient):
+        resp = await client.post("/api/v1/auth/login", json={
+            "username": "nobody",
+            "password": "123456",
+        })
+        assert resp.status_code == 401
+
+    async def test_login_disabled_user(self, client: AsyncClient, disabled_user):
+        resp = await client.post("/api/v1/auth/login", json={
+            "username": "disabled",
+            "password": "123456",
+        })
+        assert resp.status_code == 403
+
+
+class TestGetMe:
+    async def test_get_me_no_token(self, client: AsyncClient):
+        resp = await client.get("/api/v1/auth/me")
+        assert resp.status_code in (401, 403)
+
+    async def test_get_me_valid_token(self, client: AsyncClient, admin_user, admin_token):
+        resp = await client.get(
+            "/api/v1/auth/me",
+            headers={"Authorization": f"Bearer {admin_token}"},
+        )
+        assert resp.status_code == 200
+        data = resp.json()
+        assert data["username"] == "admin"
+        assert "id" in data
+        assert "role" in data
+
+    async def test_get_me_expired_token(self, client: AsyncClient, admin_user):
+        from datetime import timedelta
+        expired = create_access_token(
+            data={"sub": str(admin_user.id), "role": admin_user.role},
+            expires_delta=timedelta(seconds=-1),
+        )
+        resp = await client.get(
+            "/api/v1/auth/me",
+            headers={"Authorization": f"Bearer {expired}"},
+        )
+        assert resp.status_code == 401
+
+
+class TestLogout:
+    async def test_logout(self, client: AsyncClient, admin_token):
+        resp = await client.post(
+            "/api/v1/auth/logout",
+            headers={"Authorization": f"Bearer {admin_token}"},
+        )
+        assert resp.status_code == 200