add style guide, ruff config, generation service extraction, remove Makefile

- Add backend/STYLE_GUIDE.md covering formatting, imports, types, docstrings,
  comments, error handling, async, logging, and naming conventions
- Add pyproject.toml with ruff linter/formatter config (ERA, FIX, isort, pyupgrade)
- Extract generation service (Phase 3): unified run_generation() replaces three
  duplicated closures, serial queue moved to services/task_queue.py
- Delete Makefile in favor of justfile; update all references
- Add Python lint/format/test commands to justfile (check-python, fix-python, test)
- Install ruff, pytest, pytest-asyncio as dev tools in setup-python
- Update REFACTOR_PLAN.md with Phase 3 and Phase 7 completion

PATCH_NOTES.md

@@ -31,7 +31,7 @@ Two-part fix:
 
 ## Testing
 To test this fix:
-1. Build Voicebox from source: `make build`
+1. Build Voicebox from source: `just build`
 2. Disconnect from internet
 3. Try generating speech
 4. Should work without network requests
@@ -40,13 +40,13 @@ To test this fix:
 
 ```bash
 # Install dependencies
-pip install -r requirements.txt
+just setup
 
 # Build the app
-make build
+just build
 
 # Or build just the server
-make build-server
+just build-server
 ```
 
 ## Notes