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/imports/schemas.py

@@ -1,3 +1,6 @@
+from __future__ import annotations
+import uuid
+from datetime import datetime
 from pydantic import BaseModel
 from typing import Any
 
@@ -41,3 +44,26 @@ class StepUploadResponse(BaseModel):
     file_hash: str
     status: str
     matched_items: list[str] = []
+
+
+# ── Import Validation ──────────────────────────────────────────────────────
+
+class ValidationIssue(BaseModel):
+    type: str           # "missing_material" | "material_suggestion" | "no_step" | "duplicate"
+    field: str | None = None
+    value: str | None = None
+    suggestion: str | None = None
+    message: str
+
+
+class ImportValidationOut(BaseModel):
+    id: uuid.UUID
+    tenant_id: uuid.UUID | None
+    excel_path: str
+    status: str
+    summary: dict | None
+    rows: list | None
+    created_at: datetime
+    completed_at: datetime | None
+
+    model_config = {"from_attributes": True}