Scaffold SQLAlchemy models, routers, and Alembic migrations

- app/models/: Printer, Filament, PrintJob, Todo, PricingConfig with full
  SQLAlchemy 2.x mapped columns and relationships
- app/routers/: list + get-by-id endpoints for all five resources
- app/db.py: SQLAlchemy engine + SessionLocal + get_db dependency
- alembic/: env.py reads MYSQL_* env vars; 0001_initial_schema mirrors db.sql
- alembic.ini: script_location configured, URL injected at runtime

Since db.sql already created the tables, stamp the DB before running future
migrations:  alembic stamp head

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/routers/todos.py

@@ -0,0 +1,21 @@
+from fastapi import APIRouter, Depends
+from sqlalchemy.orm import Session
+
+from app.db import get_db
+from app.models.todo import Todo
+
+router = APIRouter(prefix="/todos", tags=["todos"])
+
+
+@router.get("/")
+def list_todos(db: Session = Depends(get_db)):
+    return [t.to_dict() for t in db.query(Todo).all()]
+
+
+@router.get("/{todo_id}")
+def get_todo(todo_id: int, db: Session = Depends(get_db)):
+    todo = db.query(Todo).filter(Todo.id == todo_id).first()
+    if not todo:
+        from fastapi import HTTPException
+        raise HTTPException(status_code=404, detail="Todo not found")
+    return todo.to_dict()