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/api/routers/uploads.py

@@ -56,6 +56,7 @@ class ExcelPreviewResponse(BaseModel):
     rows: list[ExcelPreviewRow]
     column_headers: list[str] = []
     template_name: str | None = None
+    validation_id: str | None = None
 
 
 # ── Finalize request models ────────────────────────────────────────────
@@ -166,6 +167,23 @@ async def upload_excel(
             },
         )
 
+    # Queue sanity-check validation task
+    validation_id: str | None = None
+    try:
+        from app.domains.imports.models import ImportValidation
+        val = ImportValidation(
+            excel_path=str(tmp_path),
+            tenant_id=getattr(user, "tenant_id", None),
+        )
+        db.add(val)
+        await db.commit()
+        await db.refresh(val)
+        validation_id = str(val.id)
+        from app.domains.imports.tasks import validate_excel_import
+        validate_excel_import.delay(validation_id, str(tmp_path), str(getattr(user, "tenant_id", "") or ""))
+    except Exception as exc:
+        pass  # validation is non-critical
+
     return ExcelPreviewResponse(
         excel_path=str(tmp_path),
         filename=file.filename or "",
@@ -181,6 +199,7 @@ async def upload_excel(
         rows=annotated_rows,
         column_headers=parsed_dict.get("column_headers", []),
         template_name=parsed_dict.get("template_name"),
+        validation_id=validation_id,
     )
 
 
@@ -409,3 +428,19 @@ async def upload_step(
         file_hash=file_hash,
         status="uploaded",
     )
+
+
+@router.get("/validations/{validation_id}")
+async def get_import_validation(
+    validation_id: uuid.UUID,
+    db: AsyncSession = Depends(get_db),
+    user: User = Depends(get_current_user),
+):
+    """Poll the result of an Excel sanity-check validation."""
+    from app.domains.imports.models import ImportValidation
+    from app.domains.imports.schemas import ImportValidationOut
+    result = await db.execute(select(ImportValidation).where(ImportValidation.id == validation_id))
+    val = result.scalar_one_or_none()
+    if not val:
+        raise HTTPException(404, detail="Validation not found")
+    return ImportValidationOut.model_validate(val)