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/main.py

@@ -16,7 +16,7 @@ from app.domains.products.router import products_router, cad_router
 from app.domains.materials.router import router as materials_router
 from app.domains.rendering.router import render_templates_router, output_types_router
 from app.domains.notifications.router import router as notifications_router
-from app.domains.billing.router import router as pricing_router
+from app.domains.billing.router import pricing_router, invoice_router
 from app.domains.tenants.router import router as tenants_router
 from app.domains.rendering.workflow_router import router as workflows_router
 from app.domains.media.router import router as media_router
@@ -73,6 +73,7 @@ app.include_router(materials_router, prefix="/api")
 app.include_router(worker_router, prefix="/api")
 app.include_router(analytics_router, prefix="/api")
 app.include_router(pricing_router, prefix="/api")
+app.include_router(invoice_router, prefix="/api")
 app.include_router(products_router, prefix="/api")
 app.include_router(output_types_router, prefix="/api")
 app.include_router(render_templates_router, prefix="/api")