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>

frontend/src/api/uploads.ts

@@ -30,6 +30,7 @@ export interface ExcelPreviewResult {
   rows: ExcelPreviewRow[]
   column_headers: string[]
   template_name: string | null
+  validation_id?: string | null
 }
 
 export interface ParsedComponent {
@@ -100,3 +101,46 @@ export async function uploadStep(file: File) {
   })
   return res.data
 }
+
+// ── Import Validation ─────────────────────────────────────────────────────
+
+export interface ValidationIssue {
+  type: 'missing_material' | 'material_suggestion' | 'no_step' | 'duplicate'
+  field: string | null
+  value: string | null
+  suggestion: string | null
+  message: string
+}
+
+export interface ValidationRow {
+  row_index: number
+  product_id: string | null
+  pim_id: string | null
+  produkt_baureihe: string | null
+  issues: ValidationIssue[]
+  status: 'ok' | 'warning' | 'error'
+}
+
+export interface ImportValidation {
+  id: string
+  tenant_id: string | null
+  excel_path: string
+  status: 'pending' | 'running' | 'completed' | 'failed'
+  summary: {
+    total: number
+    ok: number
+    warnings: number
+    errors: number
+    missing_materials: number
+    no_step: number
+    duplicates: number
+  } | null
+  rows: ValidationRow[] | null
+  created_at: string
+  completed_at: string | null
+}
+
+export async function getImportValidation(id: string): Promise<ImportValidation> {
+  const res = await api.get<ImportValidation>(`/uploads/validations/${id}`)
+  return res.data
+}