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/conftest.py

@@ -0,0 +1,139 @@
+"""Shared test fixtures for Lyria Studio backend."""
+
+import asyncio
+import os
+import sys
+from pathlib import Path
+
+import pytest
+import pytest_asyncio
+from httpx import ASGITransport, AsyncClient
+
+# ── Path setup: ensure src/ is importable ──────────────────────
+BACKEND_SRC = Path(__file__).resolve().parent.parent / "src"
+sys.path.insert(0, str(BACKEND_SRC))
+
+
+@pytest.fixture(scope="session")
+def event_loop():
+    """Create a single event loop for the entire test session."""
+    loop = asyncio.new_event_loop()
+    yield loop
+    loop.close()
+
+
+@pytest_asyncio.fixture
+async def app_client(tmp_path):
+    """Provide an httpx AsyncClient wired to the FastAPI app
+    with a fresh temp database and data directories.
+
+    Each test gets its own isolated DB and file directories.
+    """
+
+    # Create temp data directories
+    data_dir = tmp_path / "data"
+    data_dir.mkdir()
+    clips_dir = data_dir / "clips"
+    clips_dir.mkdir()
+    waveforms_dir = data_dir / "waveforms"
+    waveforms_dir.mkdir()
+    artwork_dir = data_dir / "artwork"
+    artwork_dir.mkdir()
+    test_db = data_dir / "lyria.db"
+
+    # Set env vars BEFORE import so module-level Path() calls use test dirs
+    # (image_gen.py reads ARTWORK_DIR from env, main.py uses __file__-relative paths)
+    os.environ["ARTWORK_DIR"] = str(artwork_dir)
+
+    # Import modules AFTER setting env vars
+    import image_gen as ig_mod
+    import database as db_mod
+    import waveform as wf_mod
+    import main
+
+    # Save originals for restoration
+    orig_db_path = db_mod.DB_PATH
+    orig_clips_dir = main.CLIPS_DIR
+    orig_artwork_dir = main.ARTWORK_DIR
+    orig_wf_clips = wf_mod.CLIPS_DIR
+    orig_wf_wave = wf_mod.WAVEFORM_DIR
+    orig_ig_artwork = ig_mod.ARTWORK_DIR
+
+    # Patch module-level path constants to test directories
+    db_mod.DB_PATH = test_db
+    main.CLIPS_DIR = clips_dir
+    main.ARTWORK_DIR = artwork_dir
+    wf_mod.CLIPS_DIR = clips_dir
+    wf_mod.WAVEFORM_DIR = waveforms_dir
+    ig_mod.ARTWORK_DIR = artwork_dir
+
+    # Initialize fresh database
+    await db_mod.init_db()
+
+    transport = ASGITransport(app=main.app)
+    async with AsyncClient(transport=transport, base_url="http://test") as c:
+        yield {
+            "client": c,
+            "db": test_db,
+            "clips_dir": clips_dir,
+            "waveforms_dir": waveforms_dir,
+            "artwork_dir": artwork_dir,
+            "data_dir": data_dir,
+        }
+
+    # Restore originals
+    db_mod.DB_PATH = orig_db_path
+    main.CLIPS_DIR = orig_clips_dir
+    main.ARTWORK_DIR = orig_artwork_dir
+    wf_mod.CLIPS_DIR = orig_wf_clips
+    wf_mod.WAVEFORM_DIR = orig_wf_wave
+    ig_mod.ARTWORK_DIR = orig_ig_artwork
+
+    # Clean up env var
+    del os.environ["ARTWORK_DIR"]
+
+
+# ── Helper: create a clip record + dummy MP3 ───────────────────
+
+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."""
+    import aiosqlite
+    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
+
+
+# ── Helper: create a real (ffmpeg-generated) MP3 ────────────────
+
+def create_real_mp3(output_path, duration=2):
+    """Generate a real MP3 file using ffmpeg.
+
+    Returns True if successful, False if ffmpeg is unavailable.
+    """
+    import subprocess
+    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