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/__init__.py

@@ -0,0 +1,8 @@
+from app.models.base import Base
+from app.models.filament import Filament
+from app.models.pricing import PricingConfig
+from app.models.print_job import PrintJob
+from app.models.printer import Printer
+from app.models.todo import Todo
+
+__all__ = ["Base", "Printer", "Filament", "PrintJob", "Todo", "PricingConfig"]