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/db.py

@@ -1,22 +1,33 @@
 import os
-import pymysql
+
+from sqlalchemy import create_engine, text
+from sqlalchemy.orm import Session, sessionmaker
 
 
-def _params() -> dict:
-    return {
-        "host": os.environ["MYSQL_HOST"],
-        "port": int(os.environ.get("MYSQL_PORT", 3306)),
-        "user": os.environ["MYSQL_USER"],
-        "password": os.environ["MYSQL_PASSWORD"],
-        "database": os.environ["MYSQL_DATABASE"],
-        "connect_timeout": 3,
-    }
+def _db_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']}"
+    )
+
+
+engine = create_engine(_db_url(), pool_pre_ping=True)
+SessionLocal = sessionmaker(bind=engine)
+
+
+def get_db():
+    db: Session = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
 
 
 def check_db() -> tuple[bool, str]:
     try:
-        conn = pymysql.connect(**_params())
-        conn.close()
+        with engine.connect() as conn:
+            conn.execute(text("SELECT 1"))
         return True, "ok"
     except Exception as e:
         return False, str(e)