feat: ✨ Bridge LINE Flex/bot messages

[clins1994]

Feature Request: Bridge LINE Flex/bot messages

Is your feature request related to a problem? Please describe

LINE Flex/bot messages are currently skipped entirely by the bridge. These show up as unsupported content_type=22 messages with FLEX_JSON metadata and often an ALT_TEXT fallback.

This matters because many Japanese LINE official accounts use rich messages for real service workflows like reservations, appointment reminders, coupons, delivery notices, and LIFF links.

Describe the solution you'd like

Bridge LINE content_type=22 messages instead of dropping them.

Suggested first step:

1. If ALT_TEXT exists, send it to Matrix as a fallback notice/text message.
2. If the rich message cannot be rendered and no useful fallback exists, send a short notice like: Unsupported LINE rich message. Please check the LINE app.

Richer FLEX_JSON rendering can be handled later.

Describe alternatives you've considered

• Keep dropping these messages: users miss important official-account messages.
• Fully render Flex messages immediately: useful, but more complex than a safe fallback.

Additional context

Example redacted log line:

matrix-line-1 | 2026-04-29T02:47:45.548Z DBG Skipping unsupported content type chunk_count=0 content_metadata={"ALT_TEXT":"完了通知","BOT_CHECK":"1","BOT_TRACK":"api=pu","FLEX_JSON":"{... redacted LINE Flex bubble with appointment text and action buttons ...}"} content_type=22 msg_id=[REDACTED] text=

[clins1994]

I saw some [Image unavailable — LINE media expired before it could be bridged] on chats with the business account today