Phase 1-2 (PLAN_MASTERBUS_FX_RACK_VST): tests cho render_fx_chain + API fx-render; plan HOÀN THÀNH Phase 0-4

- test_native_render: render_fx_chain builtin gain e2e (bridge --render-fx)
- test_plugin_api: POST /api/v1/plugins/fx-render builtin gain
- PLAN_MASTERBUS_FX_RACK_VST.md: cap nhat trang thai cac phase (0-4 da implement tu cac commit truoc + phan nay)

tests/test_plugin_api.py

@@ -30,6 +30,47 @@ def get_admin_token():
 
 
 class TestPluginAPI:
+    def test_fx_render_builtin_gain(self):
+        """POST /api/v1/plugins/fx-render với builtin gain (bridge --render-fx),
+        không cần VST — output WAV mới trong PROCESSED_DIR."""
+        import numpy as np
+        import soundfile as sf
+        from app.core import native_render
+        from app.config import settings
+        exe = native_render.find_bridge_exe()
+        if not exe:
+            pytest.skip("bridge exe missing")
+        token = get_admin_token()
+        if not token:
+            pytest.skip("no admin token")
+        h = {"Authorization": f"Bearer {token}"}
+        sr = 44100
+        t = np.linspace(0, 0.5, int(sr * 0.5), endpoint=False)
+        data = (0.05 * np.sin(2 * np.pi * 440 * t)).astype(np.float32)
+        os.makedirs(settings.PROCESSED_DIR, exist_ok=True)
+        inp = os.path.join(settings.PROCESSED_DIR, "fx_api_in_test.wav")
+        sf.write(inp, data, sr, subtype="FLOAT")
+        try:
+            resp = client.post("/api/v1/plugins/fx-render", headers=h, json={
+                "input_file_id": "fx_api_in_test.wav",
+                "fx_chain": [{"type": "builtin", "id": "gain", "params": {"db": 6.0}}],
+                "sample_rate": sr,
+            })
+            assert resp.status_code == 200, resp.text[-500:]
+            body = resp.json()
+            assert body["success"] is True
+            assert os.path.isfile(body["path"])
+            d, _ = sf.read(body["path"], dtype="float32", always_2d=True)
+            assert float(np.max(np.abs(d))) > 0.07  # ~2x 0.05
+        finally:
+            client.cookies.clear()  # không làm hỏng test requires_auth kế tiếp
+            for f in (inp, os.path.join(settings.PROCESSED_DIR, "fx_api_out_test.wav")):
+                if os.path.exists(f):
+                    try:
+                        os.remove(f)
+                    except Exception:
+                        pass
+
     def test_list_plugins_requires_auth(self):
         resp = client.get("/api/v1/plugins/available")
         assert resp.status_code in (401, 403)