feat: add decision API routes with tests

PUT /api/decisions/{listing_id} to set decision,
GET /api/decisions to list all user decisions,
DELETE /api/decisions/{listing_id} to remove a decision.
All 6 API route tests pass.

tests/unit/test_decision_routes.py

@@ -0,0 +1,128 @@
+"""Unit tests for decision API routes."""
+import pytest
+from httpx import ASGITransport, AsyncClient
+from sqlalchemy import Engine
+from sqlmodel import SQLModel, Session, create_engine
+
+from models.user import User
+from models.decision import ListingDecision  # noqa: F401 - needed for table creation
+from api.auth import get_current_user, User as AuthUser
+
+
+@pytest.fixture
+def decision_engine() -> Engine:
+    engine = create_engine(
+        "sqlite:///:memory:",
+        echo=False,
+        connect_args={"check_same_thread": False},
+    )
+    SQLModel.metadata.create_all(engine)
+    with Session(engine) as session:
+        session.add(User(id=1, email="test@example.com"))
+        session.commit()
+    yield engine  # type: ignore[misc]
+    SQLModel.metadata.drop_all(engine)
+
+
+@pytest.fixture
+async def client(decision_engine: Engine) -> AsyncClient:
+    import database
+    import api.app as api_app
+    import api.decision_routes as decision_routes_mod
+    import api.poi_routes as poi_routes_mod
+
+    app = api_app.app
+    mock_user = AuthUser(
+        sub="test-user-id", email="test@example.com", name="Test User"
+    )
+    app.dependency_overrides[get_current_user] = lambda: mock_user
+
+    original_db = database.engine
+    original_app = api_app.engine
+    original_decision = decision_routes_mod.engine
+    original_poi = poi_routes_mod.engine
+    database.engine = decision_engine
+    api_app.engine = decision_engine
+    decision_routes_mod.engine = decision_engine
+    poi_routes_mod.engine = decision_engine
+
+    transport = ASGITransport(app=app)
+    async with AsyncClient(transport=transport, base_url="http://test") as c:
+        yield c  # type: ignore[misc]
+
+    database.engine = original_db
+    api_app.engine = original_app
+    decision_routes_mod.engine = original_decision
+    poi_routes_mod.engine = original_poi
+    app.dependency_overrides.clear()
+
+
+class TestDecisionRoutes:
+    @pytest.mark.asyncio
+    async def test_set_decision(self, client: AsyncClient) -> None:
+        resp = await client.put(
+            "/api/decisions/100",
+            json={"decision": "liked", "listing_type": "RENT"},
+        )
+        assert resp.status_code == 200
+        data = resp.json()
+        assert data["decision"] == "liked"
+        assert data["listing_id"] == 100
+
+    @pytest.mark.asyncio
+    async def test_get_decisions(self, client: AsyncClient) -> None:
+        await client.put(
+            "/api/decisions/100",
+            json={"decision": "liked", "listing_type": "RENT"},
+        )
+        resp = await client.get("/api/decisions")
+        assert resp.status_code == 200
+        data = resp.json()
+        assert len(data) == 1
+
+    @pytest.mark.asyncio
+    async def test_delete_decision(self, client: AsyncClient) -> None:
+        await client.put(
+            "/api/decisions/100",
+            json={"decision": "liked", "listing_type": "RENT"},
+        )
+        resp = await client.delete(
+            "/api/decisions/100", params={"listing_type": "RENT"}
+        )
+        assert resp.status_code == 200
+        assert resp.json()["success"] is True
+
+    @pytest.mark.asyncio
+    async def test_delete_nonexistent_returns_404(
+        self, client: AsyncClient
+    ) -> None:
+        resp = await client.delete(
+            "/api/decisions/999", params={"listing_type": "RENT"}
+        )
+        assert resp.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_invalid_decision_returns_400(
+        self, client: AsyncClient
+    ) -> None:
+        resp = await client.put(
+            "/api/decisions/100",
+            json={"decision": "maybe", "listing_type": "RENT"},
+        )
+        assert resp.status_code == 400
+
+    @pytest.mark.asyncio
+    async def test_update_decision(self, client: AsyncClient) -> None:
+        await client.put(
+            "/api/decisions/100",
+            json={"decision": "liked", "listing_type": "RENT"},
+        )
+        resp = await client.put(
+            "/api/decisions/100",
+            json={"decision": "disliked", "listing_type": "RENT"},
+        )
+        assert resp.status_code == 200
+        assert resp.json()["decision"] == "disliked"
+        # Still only one decision
+        resp2 = await client.get("/api/decisions")
+        assert len(resp2.json()) == 1