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/alembic/versions/043_import_validations.py

@@ -0,0 +1,33 @@
+"""Add import_validations table.
+
+Revision ID: 043
+Revises: 042
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+
+revision = '043'
+down_revision = '042'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        'import_validations',
+        sa.Column('id', UUID(as_uuid=True), primary_key=True, server_default=sa.text('gen_random_uuid()')),
+        sa.Column('tenant_id', UUID(as_uuid=True), sa.ForeignKey('tenants.id', ondelete='CASCADE'), nullable=True),
+        sa.Column('excel_path', sa.Text, nullable=False),
+        sa.Column('status', sa.String(20), nullable=False, server_default='pending'),
+        sa.Column('summary', JSONB, nullable=True),
+        sa.Column('rows', JSONB, nullable=True),
+        sa.Column('created_at', sa.DateTime, nullable=False, server_default=sa.text('NOW()')),
+        sa.Column('completed_at', sa.DateTime, nullable=True),
+    )
+    op.create_index('ix_import_validations_tenant', 'import_validations', ['tenant_id'])
+    op.create_index('ix_import_validations_status', 'import_validations', ['status'])
+
+
+def downgrade():
+    op.drop_table('import_validations')