The IM renderers cut the summary of a single push at 120 characters and
listed at most 3 assets, as fixed constants. Both are now global settings
in the settings table, read by the delivery engine when it renders:
notify_summary_max_runes 0-4000, default 120
notify_assets_max_shown 0-50, default 3
0 means no truncation and an unset key keeps 120 / 3. PUT /api/settings
answers 400 for a value outside the bounds. The limits travel on
notify.Message, so the notify package still does not touch the database.
With no truncation, nothing bounded a single item in a Feishu card, and
Feishu rejects an oversized request as a whole. feishuCard now truncates
an item's text as the final backstop, the role TruncateBytes plays in
markdownBody for DingTalk and WeCom. feishuMaxCardBytes goes from 24000
to 16000: Feishu custom bots cap the request body at 20 KB, which the old
value exceeded.
Refs #168.
