feat(T04): Rentman integration – equipment sync, API endpoints, frontend updates

- Update equipment model with Rentman-compatible fields
- Add/Update equipment router endpoints for sync operations
- Enhance rentman_service with full API client logic
- Improve sync_service for bidirectional equipment sync
- Update docker-compose with Rentman env vars
- Update frontend useApi composable and nuxt config
- Update mietkatalog pages with Rentman-integrated data display
- Add images/ to .gitignore (binary assets, 36MB)

backend/app/models/equipment.py

@@ -15,12 +15,20 @@ class EquipmentCache(Base):
     description = Column(Text)
     specifications = Column(JSON)
     images = Column(JSON)
+    update_hash = Column(String(128))
     rental_price = Column(DECIMAL(10, 2))
     brand = Column(String(128))
     available = Column(Boolean, default=True)
     created_at = Column(TIMESTAMP, server_default=func.now())
     updated_at = Column(TIMESTAMP, server_default=func.now(), onupdate=func.now())
 
+    @property
+    def image_url(self) -> str | None:
+        """Return local image URL if images exist, else None."""
+        if self.images and len(self.images) > 0:
+            return f"/api/equipment/{self.id}/image"
+        return None
+
     __table_args__ = (
         Index("idx_equipment_category", "category"),
         Index("idx_equipment_name", "name"),