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/base.py

@@ -0,0 +1,6 @@
+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    def to_dict(self) -> dict:
+        return {c.name: getattr(self, c.name) for c in self.__table__.columns}