Add test suite for Lyria Studio backend

- conftest.py: isolated async test fixtures with temp DB and directories
- test_waveform.py: waveform generation, downsampling, caching, SVG output
- test_api.py: full API endpoint tests (health, models, clips CRUD,
  waveform endpoint, admin cache clearing)
- test_analysis.py: BPM detection, key detection, clip analysis
- test_models.py: request validation, model registry, prompt building

- Fixed off-by-one in waveform.py downsampling (linspace upper bound)
- All 40 tests passing

backend/tests/test_api.py

@@ -0,0 +1,222 @@
+"""Tests for Lyria Studio API endpoints."""
+
+import subprocess
+import sys
+import time
+from pathlib import Path
+
+import pytest
+import pytest_asyncio
+import aiosqlite
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent / "src"))
+
+
+async def seed_test_clip(app_client, prompt="test prompt", model="lyria-3-clip-preview", duration=30):
+    """Create a clip record in the test DB with a dummy MP3 file."""
+    from database import create_clip
+
+    clip_id = await create_clip(prompt, model, duration)
+
+    filename = f"{clip_id}.mp3"
+    mp3_path = app_client["clips_dir"] / filename
+    mp3_path.write_bytes(b"\xff\xfb\x90\x00" + b"\x00" * 4096)
+
+    async with aiosqlite.connect(app_client["db"]) as db:
+        await db.execute(
+            "UPDATE clips SET status = ?, filename = ? WHERE id = ?",
+            ("ready", filename, clip_id),
+        )
+        await db.commit()
+
+    return clip_id
+
+
+def create_real_mp3(output_path, duration=2):
+    """Generate a real MP3 file using ffmpeg. Returns True if successful."""
+    try:
+        subprocess.run(
+            [
+                "ffmpeg", "-y", "-f", "lavfi",
+                "-i", f"anullsrc=r=22050:cl=mono",
+                "-t", str(duration), "-q:a", "9",
+                str(output_path)
+            ],
+            capture_output=True, check=True,
+        )
+        return True
+    except (FileNotFoundError, subprocess.CalledProcessError):
+        return False
+
+
+class TestHealthEndpoint:
+    """Basic health check."""
+
+    @pytest.mark.asyncio
+    async def test_health_returns_ok(self, app_client):
+        response = await app_client["client"].get("/api/health")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["status"] == "ok"
+        assert "version" in data
+
+
+class TestModelsEndpoint:
+    """Model listing."""
+
+    @pytest.mark.asyncio
+    async def test_list_models(self, app_client):
+        response = await app_client["client"].get("/api/models")
+        assert response.status_code == 200
+        data = response.json()
+        assert "models" in data
+        assert "default" in data
+        assert "lyria-3-clip-preview" in data["models"]
+
+
+class TestClipsEndpoints:
+    """Clip CRUD operations."""
+
+    @pytest.mark.asyncio
+    async def test_list_clips_empty(self, app_client):
+        response = await app_client["client"].get("/api/clips")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["clips"] == []
+        assert data["total"] == 0
+
+    @pytest.mark.asyncio
+    async def test_seed_and_list_clip(self, app_client):
+        clip_id = await seed_test_clip(app_client)
+
+        response = await app_client["client"].get("/api/clips")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["total"] >= 1
+        clip = data["clips"][0]
+        assert clip["id"] == clip_id
+        assert clip["status"] == "ready"
+
+    @pytest.mark.asyncio
+    async def test_get_clip_by_id(self, app_client):
+        clip_id = await seed_test_clip(app_client, prompt="gabber kick at 180bpm")
+
+        response = await app_client["client"].get(f"/api/clips/{clip_id}")
+        assert response.status_code == 200
+        clip = response.json()
+        assert clip["id"] == clip_id
+        assert clip["prompt"] == "gabber kick at 180bpm"
+
+    @pytest.mark.asyncio
+    async def test_get_nonexistent_clip(self, app_client):
+        response = await app_client["client"].get("/api/clips/deadbeef-0000-0000-0000-000000000000")
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_rename_clip(self, app_client):
+        clip_id = await seed_test_clip(app_client)
+
+        response = await app_client["client"].patch(
+            f"/api/clips/{clip_id}",
+            json={"title": "New Title"},
+        )
+        assert response.status_code == 200
+        clip = response.json()
+        assert clip["title"] == "New Title"
+
+    @pytest.mark.asyncio
+    async def test_update_metadata(self, app_client):
+        clip_id = await seed_test_clip(app_client)
+
+        response = await app_client["client"].patch(
+            f"/api/clips/{clip_id}/metadata",
+            json={"artist": "Kagamine Rin", "album": "HARDCORE WILL NEVER DIE", "comment": "180 BPM banger"},
+        )
+        assert response.status_code == 200
+        clip = response.json()
+        assert clip["artist"] == "Kagamine Rin"
+        assert clip["album"] == "HARDCORE WILL NEVER DIE"
+
+    @pytest.mark.asyncio
+    async def test_delete_clip(self, app_client):
+        clip_id = await seed_test_clip(app_client)
+
+        response = await app_client["client"].delete(f"/api/clips/{clip_id}")
+        assert response.status_code == 200
+        assert response.json()["deleted"] is True
+
+        # Verify it's gone
+        response = await app_client["client"].get(f"/api/clips/{clip_id}")
+        assert response.status_code == 404
+
+
+class TestWaveformEndpoint:
+    """Waveform generation and caching."""
+
+    @pytest.mark.asyncio
+    async def test_waveform_returns_404_for_missing_clip(self, app_client):
+        response = await app_client["client"].get(
+            "/api/clips/deadbeef-0000-0000-0000-000000000000/waveform"
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_waveform_returns_svg_for_existing_clip(self, app_client):
+        clip_id = await seed_test_clip(app_client)
+        mp3_path = app_client["clips_dir"] / f"{clip_id}.mp3"
+
+        if not create_real_mp3(mp3_path, duration=2):
+            pytest.skip("ffmpeg not available on this system")
+
+        response = await app_client["client"].get(f"/api/clips/{clip_id}/waveform")
+        assert response.status_code == 200
+        assert response.headers["content-type"] == "image/svg+xml"
+        assert "Cache-Control" in response.headers
+        assert "max-age=86400" in response.headers["Cache-Control"]
+
+        # Verify SVG content
+        content = response.text
+        assert "<svg" in content
+        assert "</svg>" in content
+
+    @pytest.mark.asyncio
+    async def test_waveform_caches_on_second_request(self, app_client):
+        clip_id = await seed_test_clip(app_client)
+        mp3_path = app_client["clips_dir"] / f"{clip_id}.mp3"
+
+        if not create_real_mp3(mp3_path, duration=2):
+            pytest.skip("ffmpeg not available on this system")
+
+        # First request — may generate on demand
+        response1 = await app_client["client"].get(f"/api/clips/{clip_id}/waveform")
+        assert response1.status_code == 200
+
+        # Second request — should be served from disk cache
+        start = time.time()
+        response2 = await app_client["client"].get(f"/api/clips/{clip_id}/waveform")
+        elapsed = time.time() - start
+
+        assert response2.status_code == 200
+        assert response2.text == response1.text  # Same SVG content
+        assert elapsed < 1.0, f"Cached waveform took {elapsed:.3f}s — too slow"
+
+
+class TestClearWaveformsEndpoint:
+    """Admin waveform cache clearing."""
+
+    @pytest.mark.asyncio
+    async def test_clear_waveforms_returns_count(self, app_client):
+        waveforms_dir = app_client["waveforms_dir"]
+
+        # Create a couple fake waveform files
+        (waveforms_dir / "test1.svg").write_text("<svg>1</svg>")
+        (waveforms_dir / "test2.svg").write_text("<svg>2</svg>")
+
+        response = await app_client["client"].post("/api/admin/clear-waveforms")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["cleared"] == 2
+
+        # Verify they're gone
+        remaining = list(waveforms_dir.glob("*.svg"))
+        assert len(remaining) == 0