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/filament.py

@@ -0,0 +1,38 @@
+import enum
+from datetime import datetime
+from decimal import Decimal
+
+from sqlalchemy import DateTime, Enum, Numeric, String, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base
+
+
+class Material(enum.Enum):
+    PLA = "PLA"
+    PETG = "PETG"
+    ABS = "ABS"
+    ASA = "ASA"
+    TPU = "TPU"
+    Nylon = "Nylon"
+    PC = "PC"
+    Other = "Other"
+
+
+class Filament(Base):
+    __tablename__ = "filaments"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    brand: Mapped[str | None] = mapped_column(String(100))
+    material: Mapped[Material] = mapped_column(Enum(Material), default=Material.PLA)
+    color: Mapped[str | None] = mapped_column(String(100))
+    color_hex: Mapped[str | None] = mapped_column(String(7))
+    weight_total_g: Mapped[Decimal] = mapped_column(Numeric(8, 2))
+    weight_remaining_g: Mapped[Decimal] = mapped_column(Numeric(8, 2))
+    price_per_kg_eur: Mapped[Decimal | None] = mapped_column(Numeric(8, 2))
+    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()
+    )
+
+    jobs: Mapped[list["PrintJob"]] = relationship(back_populates="filament")