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/src/waveform.py

@@ -58,7 +58,7 @@ async def generate_waveform_svg(clip_id: str, filename: str) -> str:
     if len(audio_data) > DOWNSAMPLE_TARGET:
         # Simple slice-based downsampling (nearest-sample, no aliasing
         # concerns at this resolution — we're making bars, not audio)
-        indices = np.linspace(0, len(audio_data), DOWNSAMPLE_TARGET, dtype=int)
+        indices = np.linspace(0, len(audio_data) - 1, DOWNSAMPLE_TARGET, dtype=int)
         audio_data = audio_data[indices]
 
     # ── WAVEFORM COMPUTATION ──────────────────────────────────────