feat: add multi-destination backup support (FTP, SFTP, GDrive, OneDrive, Azure, B2, WebDAV)

[tarai-dl]

Summary

Adds support for multiple backup destination types beyond S3-compatible storage.

New Destination Types:

Type	Credentials
S3 (existing)	accessKey, secretAccessKey, bucket, region, endpoint
FTP	host, port, username, password
SFTP	host, port, username, password
Google Drive	client_id, client_secret, OAuth token
OneDrive	client_id, client_secret, OAuth token
Azure Blob	account name, access key
Backblaze B2	account ID, application key
WebDAV	URL, username, password

Implementation:

• Schema: Added destinationType, host, port, username, password, token, configOptions fields to destination table
• Utils: Added getRcloneFlags(destination) - generates rclone flags for any destination type
• Utils: Added getRcloneDestPath(destination) - generates type-aware rclone destination path
• Backward compatible: S3 destinations continue to work via existing getS3Credentials()

Usage (rclone):

All types use rclone under the hood. The new functions generate the appropriate rclone flags:

// FTP example
const flags = getRcloneFlags(destination);
// → ['--ftp-host="ftp.example.com"', '--ftp-port="21"', ...]

const destPath = getRcloneDestPath(destination);
// → ':ftp:/backups/'

Next Steps (follow-up PRs):

• Database migration for new fields
• UI form updates to show type-specific credential fields
• Update all backup upload paths to use getRcloneFlags instead of hardcoded S3

Closes #168

/claim #168