feat(F-G-H-I): STL cache, invoices, import validation, notification settings

Phase F — STL Hash Cache:
- Migration 041: step_file_hash column on cad_files
- cache_service.py: SHA256 hash + MinIO-backed STL cache (check/store)
- render_step_thumbnail: compute+persist hash before render
- generate_stl_cache: check MinIO cache before cadquery conversion, store after

Phase G — Invoices:
- Migration 042: invoices + invoice_lines tables with RLS
- Invoice/InvoiceLine models + schemas
- billing service: generate_invoice_number (INV-YYYY-NNNN), create/list/get/delete/PDF
- WeasyPrint PDF generation; backend Dockerfile + pyproject.toml deps
- invoice_router with 6 endpoints; registered in main.py
- frontend: Billing.tsx page + api/billing.ts; route + nav link

Phase H — Import Sanity Check:
- Migration 043: import_validations table
- ImportValidation model + schemas
- run_sanity_check: material fuzzy-match (cutoff=0.8), STEP availability, duplicate detection
- validate_excel_import Celery task (queue: step_processing)
- uploads.py: create ImportValidation on /excel, fire task, expose GET /validations/{id}
- frontend: Upload.tsx polling ValidationDialog with Ampel status indicators

Phase I — Notification Settings:
- Migration 044: notification_configs table (user×event×channel toggles)
- NotificationConfig model + seeds (in_app=true, email=false)
- get/upsert/reset config endpoints on /notifications/config
- frontend: NotificationSettings.tsx page + api/notifications.ts extensions

Infrastructure:
- docker-compose.yml: add worker-thumbnail service (concurrency=1, Q=thumbnail_rendering)
- Fix Dockerfile: libgdk-pixbuf-2.0-0 (correct Debian bookworm package name)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/billing/models.py

@@ -1,7 +1,7 @@
 import uuid
-from datetime import datetime
+from datetime import date, datetime
 from decimal import Decimal
-from sqlalchemy import String, Boolean, DateTime, Text, Numeric, Integer, UniqueConstraint, Index, ForeignKey
+from sqlalchemy import String, Boolean, Date, DateTime, Text, Numeric, Integer, UniqueConstraint, Index, ForeignKey
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID
 from app.database import Base
@@ -31,3 +31,38 @@ class PricingTier(Base):
         UniqueConstraint("category_key", "quality_level", name="uq_pricing_tier"),
         Index("ix_pricing_tiers_category_key", "category_key"),
     )
+
+
+class Invoice(Base):
+    __tablename__ = "invoices"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True)
+    invoice_number: Mapped[str] = mapped_column(String(20), nullable=False, unique=True)
+    status: Mapped[str] = mapped_column(String(20), nullable=False, default="draft")
+    issued_at: Mapped[date | None] = mapped_column(Date, nullable=True)
+    due_at: Mapped[date | None] = mapped_column(Date, nullable=True)
+    total_net: Mapped[Decimal | None] = mapped_column(Numeric(12, 2), nullable=True)
+    total_vat: Mapped[Decimal | None] = mapped_column(Numeric(12, 2), nullable=True)
+    vat_rate: Mapped[Decimal] = mapped_column(Numeric(5, 4), nullable=False, default=Decimal("0.19"))
+    currency: Mapped[str] = mapped_column(String(3), nullable=False, default="EUR")
+    notes: Mapped[str | None] = mapped_column(Text, nullable=True)
+    pdf_key: Mapped[str | None] = mapped_column(Text, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+    lines: Mapped[list["InvoiceLine"]] = relationship("InvoiceLine", back_populates="invoice", cascade="all, delete-orphan")
+
+
+class InvoiceLine(Base):
+    __tablename__ = "invoice_lines"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    invoice_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("invoices.id", ondelete="CASCADE"), nullable=False)
+    order_line_id: Mapped[uuid.UUID | None] = mapped_column(UUID(as_uuid=True), ForeignKey("order_lines.id", ondelete="SET NULL"), nullable=True)
+    description: Mapped[str] = mapped_column(Text, nullable=False)
+    quantity: Mapped[int] = mapped_column(Integer, nullable=False, default=1)
+    unit_price: Mapped[Decimal | None] = mapped_column(Numeric(10, 2), nullable=True)
+    total: Mapped[Decimal | None] = mapped_column(Numeric(10, 2), nullable=True)
+
+    invoice: Mapped["Invoice"] = relationship("Invoice", back_populates="lines")