Sync README project tree, DB schema and script docs with the code

[gistrec]

Docs-only: touches README.md and nothing else. Every fix below was verified against the code before changing.

• Known issues (mysqlclient): there is no Homebrew formula libmysqlclient-dev; the macOS fix is brew install mysql-client. The apt package name is correct and unchanged.
• Admin scripts: ADMIN_TELEGRAM_ID is not read from .env — it is a constant in scripts/broadcast.py (refund.py imports it). Reworded, with a note to change the admin ids when deploying your own copy.
• Project structure: added the missing handlers/{telegram,max}/retranscribe.py, schedulers/poller.py, schedulers/expire_pending.py, messengers/common.py and config.py, plus a Features bullet for the free wrong-language re-transcription. schedulers/ffmpeg.py is a 0-byte file that nothing imports, so it is intentionally not listed (looks like a leftover — candidate for deletion).
• Database schema: synced with database/models.py — transcriptions.id is INTEGER (not BIGINT), the payments index is idx_payments_user_recent (user_id, user_platform, id), and refinements.task_type has no default.
• Environment variables: the bot never reads TELEGRAM_API_ID/TELEGRAM_API_HASH — only the local Bot API server docker run command uses them, so putting them in .env does nothing; the table now says where they actually go.
• broadcast.py dry run does send a preview to the admin ids — the comment no longer claims it "sends nothing".
• Formatting: joined two hard-wrapped paragraphs into single lines and moved the pm2 fenced block out of the numbered list.