build(backend): manage dependencies with uv

BREAKING CHANGE: apps/backend/requirements.txt is no longer the backend dependency source. Use uv sync and uv run python main.py for backend setup and startup.

tests/test_main_manager.py

@@ -0,0 +1,61 @@
+from __future__ import annotations
+
+import builtins
+import io
+import importlib.util
+import types
+import unittest
+from contextlib import redirect_stderr
+from pathlib import Path
+from tempfile import TemporaryDirectory
+from unittest.mock import Mock, patch
+
+MAIN_PATH = Path(__file__).resolve().parents[1] / "main.py"
+MAIN_SPEC = importlib.util.spec_from_file_location("main", MAIN_PATH)
+assert MAIN_SPEC is not None and MAIN_SPEC.loader is not None
+main = importlib.util.module_from_spec(MAIN_SPEC)
+MAIN_SPEC.loader.exec_module(main)
+
+
+class BackendManagerUvTests(unittest.TestCase):
+    def test_backend_check_missing_dependency_points_to_uv_sync(self) -> None:
+        original_import = builtins.__import__
+
+        def fake_import(name, globals=None, locals=None, fromlist=(), level=0):
+            if name == "backend.main":
+                raise ModuleNotFoundError("No module named 'fastapi'", name="fastapi")
+            return original_import(name, globals, locals, fromlist, level)
+
+        args = types.SimpleNamespace(check=True)
+        stderr = io.StringIO()
+
+        with patch("builtins.__import__", side_effect=fake_import), redirect_stderr(stderr):
+            exit_code = main.run_backend(args)
+
+        self.assertEqual(exit_code, 2)
+        self.assertIn("uv sync", stderr.getvalue())
+        self.assertNotIn("pip install -r", stderr.getvalue())
+
+    def test_backend_daemon_uses_uv_run_python(self) -> None:
+        args = types.SimpleNamespace(host="127.0.0.1", port=8000, log_level="info")
+        proc = Mock(pid=12345)
+
+        with TemporaryDirectory() as tmp_dir:
+            tmp = Path(tmp_dir)
+            with (
+                patch.object(main, "BACKEND_PID", tmp / "backend.pid"),
+                patch.object(main, "BACKEND_LOG", tmp / "backend.log"),
+                patch.object(main, "LOGS_DIR", tmp),
+                patch("subprocess.Popen", return_value=proc) as popen,
+            ):
+                exit_code = main.start_backend_daemon(args)
+
+        self.assertEqual(exit_code, 0)
+        cmd = popen.call_args.args[0]
+        self.assertEqual(Path(cmd[0]).name, "uv")
+        self.assertEqual(cmd[1:3], ["run", "python"])
+        self.assertEqual(cmd[3:5], [str(main.REPO_ROOT / "main.py"), "backend"])
+
+
+if __name__ == "__main__":
+    unittest.main()