feat: incremental Telegram media backup

Single-file tool that downloads photos and documents from a Telegram
chat and resumes where it left off.

Durability:
- process oldest-to-newest and persist last_id after every message, so
  an interrupted run resumes instead of rescanning
- write state.json via temp file + rename; a torn write would otherwise
  leave unparseable JSON and block the next run
- download to a .part file and rename on success, so a killed run cannot
  leave a truncated file that the exists() check treats as complete
- retry transient errors with exponential backoff alongside the existing
  FloodWait handling, and skip permanently failed files rather than
  stalling the run

Usability:
- share one session across all archive dirs, so a login covers every
  chat instead of one per directory; migrate an existing per-archive
  session in place rather than forcing re-authentication
- create the config template before argument parsing, since --target was
  required but unknowable before the API keys were set
- add --list-chats to print dialog IDs and usernames
- convert numeric targets to int; Telethon resolves a numeric string as
  a username and never consults the entity cache

Hardening:
- strip path components from sender-controlled filenames
- create config and session files 0600 in a 0700 directory

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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