Add optional original-quality image uploads

[silencoo]

What changed

• Add an opt-in Preserve original image quality checkbox to the current homepage.
• Send opted-in images through Telegram sendDocument instead of sendPhoto.
• Keep the existing photo mode as the default for backwards compatibility.
• Snapshot the selected mode when each file enters the upload queue.
• Reject original-quality Telegram images above 20 MB, because the Bot API getFile path used to serve uploads only supports files up to 20 MB.
• Hide the Telegram-specific option when R2 storage is selected, since R2 already stores the original bytes.
• Document the imageUploadMode=document multipart API field in both READMEs.

Why

Telegram optimizes regular photo messages. Sending an image as a document preserves the uploaded file for users who prefer original quality, while leaving existing deployments and API clients unchanged unless they opt in.

Validation

• npm test — 102 passing
• Checked the homepage at desktop and 390 px mobile widths
• Verified the checkbox is keyboard-accessible and does not trigger the surrounding file picker
• Frontend audit reports only pre-existing findings in legacy admin/e2e files

Telegram references:

• Bot API: sendDocument
• Bots FAQ: getFile 20 MB download limit