feat: add document/file forwarding support

[AmaLS367]

What

Adds support for forwarding documents (PDF, DOCX, ZIP, and any other file type) sent to the bot as email attachments.

Why

Users were unable to forward files to their inbox. Sending a PDF would result in it being silently ignored by the bot.

Closes #352

How

Added a send_document handler in bot.py following the same pattern as the existing send_photo handler:

• Fetches the file via Telegram API
• Downloads it into memory
• Preserves the original filename from message.document.file_name
• Dispatches send_file Celery task (already supports arbitrary file attachments)
• Sends a 👌 reaction on success

Registered with filters.Document.ALL so it catches all document types, not just PDFs.

Changes

• src/bot.py — new send_document handler + handler registration

[f213]

@AmaLS367 i opened CI workflows for you. After you fix the linters, we can merge this

[AmaLS367]

Fixed the mypy errors by adding an assert. Could you re-approve the CI run? Thanks!

[f213]

I re-approved CI, but asserts are not a good idea in the production code. May be we should add a #type: ignore?

[AmaLS367]

Okay, thanks. I agree that keeping the assert in production code is not ideal.

I think adding a new # type: ignore here would also be a bit worse, since we already use small typed Update wrappers in this project. I updated it in that style instead: added a typed DocumentMessageUpdate for document messages and changed the handler to use it, so mypy knows message.document exists without a runtime assert.

Could you re-approve the CI run when you have a chance?