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/notifications/service.py

@@ -7,7 +7,7 @@ import logging
 import uuid
 from datetime import datetime
 
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, select
 from sqlalchemy.orm import Session
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -82,3 +82,91 @@ def emit_notification_sync(
             session.commit()
     except Exception:
         logger.exception("Failed to emit notification (sync)")
+
+
+# ── Notification config helpers ─────────────────────────────────────────────
+
+def _is_channel_enabled_sync(user_id: str | None, event_type: str, channel: str) -> bool:
+    """Check if a notification channel is enabled for a user (sync, for Celery)."""
+    if not user_id:
+        return channel == "in_app"  # default: in_app on, email off
+    engine = _get_engine()
+    from app.domains.notifications.models import NotificationConfig
+    with Session(engine) as session:
+        cfg = session.execute(
+            select(NotificationConfig).where(
+                NotificationConfig.user_id == user_id,
+                NotificationConfig.event_type == event_type,
+                NotificationConfig.channel == channel,
+            )
+        ).scalar_one_or_none()
+        if cfg is None:
+            return channel == "in_app"  # default
+        return cfg.enabled
+
+
+def send_email_notification_stub(
+    *,
+    to_user_id: str | None,
+    event_type: str,
+    subject: str,
+    body: str,
+) -> None:
+    """Email notification stub — logs only, email sending not yet active."""
+    logger.info(
+        "[EMAIL STUB] Would send email to user=%s event=%s subject=%s",
+        to_user_id, event_type, subject
+    )
+
+
+async def get_notification_configs(db: AsyncSession, user_id: uuid.UUID) -> list:
+    from app.domains.notifications.models import NotificationConfig
+    from sqlalchemy import select as sa_select
+    result = await db.execute(
+        sa_select(NotificationConfig).where(NotificationConfig.user_id == user_id)
+        .order_by(NotificationConfig.event_type, NotificationConfig.channel)
+    )
+    return list(result.scalars().all())
+
+
+async def upsert_notification_config(
+    db: AsyncSession,
+    user_id: uuid.UUID,
+    event_type: str,
+    channel: str,
+    enabled: bool,
+) -> object:
+    from app.domains.notifications.models import NotificationConfig
+    from sqlalchemy import select as sa_select
+    result = await db.execute(
+        sa_select(NotificationConfig).where(
+            NotificationConfig.user_id == user_id,
+            NotificationConfig.event_type == event_type,
+            NotificationConfig.channel == channel,
+        )
+    )
+    cfg = result.scalar_one_or_none()
+    if cfg is None:
+        cfg = NotificationConfig(user_id=user_id, event_type=event_type, channel=channel, enabled=enabled)
+        db.add(cfg)
+    else:
+        cfg.enabled = enabled
+    await db.commit()
+    await db.refresh(cfg)
+    return cfg
+
+
+async def reset_notification_configs(db: AsyncSession, user_id: uuid.UUID) -> list:
+    from app.domains.notifications.models import NotificationConfig, NotificationEvent
+    from sqlalchemy import delete as sa_delete
+    await db.execute(sa_delete(NotificationConfig).where(NotificationConfig.user_id == user_id))
+    configs = []
+    for event in NotificationEvent.ALL:
+        for channel, default_enabled in [("in_app", True), ("email", False)]:
+            cfg = NotificationConfig(
+                user_id=user_id, event_type=event, channel=channel, enabled=default_enabled
+            )
+            db.add(cfg)
+            configs.append(cfg)
+    await db.commit()
+    return configs