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>

alembic/env.py

@@ -0,0 +1,54 @@
+import os
+from logging.config import fileConfig
+
+from sqlalchemy import engine_from_config, pool
+
+from alembic import context
+
+config = context.config
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+from app.models.base import Base
+import app.models  # noqa: F401 — registers all models with Base.metadata
+
+target_metadata = Base.metadata
+
+
+def get_url() -> str:
+    return (
+        f"mysql+pymysql://{os.environ['MYSQL_USER']}:{os.environ['MYSQL_PASSWORD']}"
+        f"@{os.environ['MYSQL_HOST']}:{os.environ.get('MYSQL_PORT', 3306)}"
+        f"/{os.environ['MYSQL_DATABASE']}"
+    )
+
+
+def run_migrations_offline() -> None:
+    context.configure(
+        url=get_url(),
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    config.set_main_option("sqlalchemy.url", get_url())
+    connectable = engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+    with connectable.connect() as connection:
+        context.configure(connection=connection, target_metadata=target_metadata)
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()