Two unrelated tweaks grouped into one PR to keep churn low.
## Ports
Backend 8000 → 3900, Vite dev 5173 → 3901, 3902 reserved for future
IPC. Port 8000 conflicts with Django/Rails/Jupyter/Airflow on most
dev machines; the uncommon 3900 range dodges that. Touched:
- frontend/src-tauri/src/lib.rs (BACKEND_PORT)
- frontend/src-tauri/tauri.conf.json (devUrl)
- frontend/vite.config.js (server.port)
- frontend/src/api/client.ts (hardcoded API base)
- frontend/src/App.jsx (PREVIEW_API fallback)
- backend/main.py (CORS allowlist + uvicorn.run default)
Rust sidecar launcher and FastAPI uvicorn port stay in sync via the
`BACKEND_PORT` constant + explicit port=3900.
## CI caches
Build time shaves across ci.yml and release.yml:
- `astral-sh/setup-uv@v3` → `enable-cache: true` keyed on uv.lock
(~45 s saved per run after uv.lock stabilises)
- `awalsh128/cache-apt-pkgs-action` for ffmpeg (~25 s saved)
- `actions/cache@v4` on `~/.bun/install/cache` keyed on bun.lock
(~15 s saved; applied to both test gate and build matrix)
Expected warm test job: ~45-60 s (was ~2-3 min).
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
