feat(images): backend catalog + mirror model + download API/worker

Phase 1 of VM image mirror feature:
- StorageMixin.download_url (PVE download-url/aria2) + get_download_target_storages
- builtin curated cloud-image catalog (Ubuntu/Debian/Rocky/Alma, amd64+arm64)
- ImageMirror model for admin custom mirrors
- /proxmox/api/images: catalog, lxc-templates proxy, storages, download
- _do_image_download_sync worker with log-tailing progress (DeployTask)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/models/catalog.py

@@ -0,0 +1,54 @@
+from .base import *
+
+
+class ImageMirror(Base):
+    """
+    Кастомный источник образов (зеркало), добавленный администратором.
+    Дополняет встроенный curated-каталог (app/catalog/builtin.py).
+
+    kind:
+        'qcow2'  — cloud-образ ВМ (url обязателен)
+        'vztmpl' — LXC-шаблон (url ИЛИ template — имя файла из репозитория)
+    """
+    __tablename__ = "image_mirrors"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String(150), nullable=False)
+    kind = Column(String(20), nullable=False, default='qcow2', index=True)
+    os = Column(String(50), nullable=True)
+    version = Column(String(50), nullable=True)
+    arch = Column(String(20), nullable=False, default='amd64')
+
+    url = Column(String(1000), nullable=True)
+    template = Column(String(255), nullable=True)
+
+    checksum = Column(String(150), nullable=True)
+    checksum_algorithm = Column(String(20), nullable=True)
+
+    description = Column(Text, nullable=True)
+    enabled = Column(Boolean, default=True, nullable=False)
+
+    created_by = Column(Integer, ForeignKey("users.id", ondelete="SET NULL"), nullable=True)
+    created_at = Column(DateTime(timezone=True), server_default=func.now(), nullable=False)
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now(), nullable=False)
+
+    def to_dict(self) -> dict:
+        return {
+            'id': f'mirror-{self.id}',
+            'mirror_id': self.id,
+            'source': 'mirror',
+            'kind': self.kind,
+            'os': self.os,
+            'version': self.version,
+            'arch': self.arch,
+            'name': self.name,
+            'url': self.url,
+            'template': self.template,
+            'checksum': self.checksum,
+            'checksum_algorithm': self.checksum_algorithm,
+            'description': self.description,
+            'enabled': self.enabled,
+        }
+
+    def __repr__(self):
+        return f"<ImageMirror(id={self.id}, name='{self.name}', kind='{self.kind}')>"