chore: unique ports (3900/3901) + broader CI caches (#20)

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>

.github/workflows/ci.yml

@@ -30,8 +30,13 @@ jobs:
         with:
           python-version: "3.11"
 
+      # enable-cache persists ~/.cache/uv across runs, keyed on uv.lock —
+      # turns `uv sync` from ~45 s cold to ~5 s warm.
       - name: Install uv
         uses: astral-sh/setup-uv@v3
+        with:
+          enable-cache: true
+          cache-dependency-glob: "uv.lock"
 
       # Node 22 is needed for --experimental-strip-types so node:test can
       # import .ts files directly from frontend/src/api/*.
@@ -43,10 +48,12 @@ jobs:
       - name: Setup Bun
         uses: oven-sh/setup-bun@v1
 
+      # apt install ffmpeg is ~30 s every run; cache the resolved .debs.
       - name: System deps (ffmpeg)
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y ffmpeg
+        uses: awalsh128/cache-apt-pkgs-action@latest
+        with:
+          packages: ffmpeg
+          version: 1.0
 
       - name: Install Python deps
         run: uv sync
@@ -54,6 +61,16 @@ jobs:
       - name: Run pytest
         run: uv run pytest tests/ -q --tb=short
 
+      # Cache ~/.bun/install/cache keyed on bun.lock — `bun install` drops
+      # from ~15 s cold to near-instant on warm cache.
+      - name: Cache bun deps
+        uses: actions/cache@v4
+        with:
+          path: ~/.bun/install/cache
+          key: ${{ runner.os }}-bun-${{ hashFiles('frontend/bun.lock', 'bun.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-bun-
+
       - name: Install frontend deps
         working-directory: frontend
         run: bun install

.github/workflows/release.yml

@@ -50,8 +50,12 @@ jobs:
         with:
           python-version: "3.11"
 
+      # enable-cache persists ~/.cache/uv keyed on uv.lock.
       - name: Install uv
         uses: astral-sh/setup-uv@v3
+        with:
+          enable-cache: true
+          cache-dependency-glob: "uv.lock"
 
       # Node 22 is needed for --experimental-strip-types so node:test can
       # import .ts files directly from frontend/src/api/*.
@@ -63,13 +67,13 @@ jobs:
       - name: Setup Bun
         uses: oven-sh/setup-bun@v1
 
-      # Backend tests need ffmpeg (subprocess calls in fixtures) + the minimal
-      # apt deps pydub/imageio pull in. Model weights are mocked so no HF
-      # downloads happen.
+      # Backend tests need ffmpeg (subprocess calls in fixtures). Cache the
+      # resolved .debs so warm runs skip the apt-get update + install.
       - name: System deps (ffmpeg)
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y ffmpeg
+        uses: awalsh128/cache-apt-pkgs-action@latest
+        with:
+          packages: ffmpeg
+          version: 1.0
 
       - name: Install Python deps
         run: uv sync
@@ -77,6 +81,14 @@ jobs:
       - name: Run pytest
         run: uv run pytest tests/ -q --tb=short
 
+      - name: Cache bun deps
+        uses: actions/cache@v4
+        with:
+          path: ~/.bun/install/cache
+          key: ${{ runner.os }}-bun-${{ hashFiles('frontend/bun.lock', 'bun.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-bun-
+
       - name: Install frontend deps
         working-directory: frontend
         run: bun install
@@ -170,6 +182,14 @@ jobs:
             libasound2-dev ffmpeg
 
       # ── Frontend build ─────────────────────────────────────────────────
+      - name: Cache bun deps
+        uses: actions/cache@v4
+        with:
+          path: ~/.bun/install/cache
+          key: ${{ runner.os }}-bun-${{ hashFiles('frontend/bun.lock', 'bun.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-bun-
+
       - name: Install frontend deps
         working-directory: frontend
         run: bun install