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

backend/services/task_queue.py

@@ -0,0 +1,48 @@
+"""
+Serial generation queue — ensures only one TTS inference runs at a time
+to avoid GPU contention.
+"""
+
+import asyncio
+import traceback
+
+# Keep references to fire-and-forget background tasks to prevent GC
+_background_tasks: set = set()
+
+# Generation queue — serializes TTS inference to avoid GPU contention
+_generation_queue: asyncio.Queue = None  # type: ignore  # initialized at startup
+
+
+def create_background_task(coro) -> asyncio.Task:
+    """Create a background task and prevent it from being garbage collected."""
+    task = asyncio.create_task(coro)
+    _background_tasks.add(task)
+    task.add_done_callback(_background_tasks.discard)
+    return task
+
+
+async def _generation_worker():
+    """Worker that processes generation tasks one at a time."""
+    while True:
+        coro = await _generation_queue.get()
+        try:
+            await coro
+        except Exception:
+            traceback.print_exc()
+        finally:
+            _generation_queue.task_done()
+
+
+def enqueue_generation(coro):
+    """Add a generation coroutine to the serial queue."""
+    _generation_queue.put_nowait(coro)
+
+
+def init_queue():
+    """Initialize the generation queue and start the worker.
+
+    Must be called once during application startup (inside a running event loop).
+    """
+    global _generation_queue
+    _generation_queue = asyncio.Queue()
+    create_background_task(_generation_worker())