security: encrypt sensitive DB fields, clean up dynamic import

Encrypt Proxmox server passwords and VM cloud-init passwords in the database:
- Add backend/app/crypto.py: Fernet-based encrypt/decrypt helpers + EncryptedString
  SQLAlchemy TypeDecorator (transparent to all existing code, no API changes)
- Add FERNET_KEY config field (env var); without it values stay plaintext for
  backward compatibility, with a logged warning
- Update models.py: ProxmoxServer.password and VMInstance.cloud_init_password
  now use EncryptedString(255) instead of String(255)
- Add migration 14 in migrations/migrations.py: idempotent re-encryption of
  existing plaintext rows (skipped when FERNET_KEY is not set)

CSRF analysis:
- /api/auth/change-password is protected by JWT Bearer token (Authorization header)
- Browsers cannot auto-include Authorization headers cross-site → CSRF is not
  applicable here; added explanatory comment

Improve migration loading:
- Add migrations/__init__.py to make it a proper Python package
- Replace fragile importlib dynamic loading in main.py with a standard import

https://claude.ai/code/session_019rK2wvaWg5zXFwmAPPEB7R

backend/app/config.py

@@ -53,6 +53,11 @@ class Settings(BaseSettings):
     # Default admin password — MUST be changed via env var in production
     ADMIN_PASSWORD: str = Field(default="admin123", env="ADMIN_PASSWORD")
 
+    # Fernet key for encrypting sensitive DB fields (Proxmox passwords, cloud-init passwords).
+    # Generate with: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
+    # If not set, values are stored as plaintext (backward-compatible but insecure).
+    FERNET_KEY: Optional[str] = Field(default=None, env="FERNET_KEY")
+
     # Timezone
     TZ: str = Field(default="UTC", env="TZ")