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/api/auth.py

@@ -418,6 +418,10 @@ def delete_user(
 
 
 @router.post("/api/auth/change-password")
+# Note: no explicit CSRF token needed here.
+# This endpoint is protected by JWT Bearer token (Authorization header).
+# Browsers cannot automatically include the Authorization header in cross-site requests,
+# so CSRF attacks are not possible — the attacker cannot obtain or inject the token.
 def change_password(
     old_password: str = Form(...),
     new_password: str = Form(...),