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/models/todo.py

@@ -0,0 +1,21 @@
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, ForeignKey, String, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base
+
+
+class Todo(Base):
+    __tablename__ = "todos"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    title: Mapped[str] = mapped_column(String(255))
+    done: Mapped[bool] = mapped_column(Boolean, default=False)
+    printer_id: Mapped[int | None] = mapped_column(ForeignKey("printers.id"))
+    created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime, server_default=func.now(), onupdate=func.now()
+    )
+
+    printer: Mapped["Printer | None"] = relationship(back_populates="todos")