fix: reconcile stale broadcast channel name from QR invite

[bolstad]

Closes #8250

Problem

When a user joins a broadcast channel via a QR invite link
(https://i.delta.chat/#...), the joining device displays the channel
name taken from the link's b= parameter and never reconciles it
with the channel's actual name. If the two differ (e.g. the channel was
renamed after the link was generated, or an older link was shared), the
joining device shows the wrong name permanently — even after receiving
messages and membership events from the channel. Every other client
shows the correct name.

Root cause

1. check_qr parses the link's b= parameter into
   Qr::AskJoinBroadcast { name }.
2. On join, joining_chat_id() creates the local InBroadcast chat
   with that name via create_multiuser_record(); the chat's
   Param::GroupNameTimestamp is left unset (0).
3. create_out_broadcast_ex() creates the owner's OutBroadcast chat
   without setting Param::GroupNameTimestamp. Channels are always
   promoted, so the first-message promotion path in chat.rs that sets
   the timestamp for Group | OutBroadcast is never reached.
4. Consequently mimefactory emits Chat-Group-Name but never
   Chat-Group-Name-Timestamp for channel messages.
5. apply_chat_name_avatar_and_description_changes() only enters the
   name-update block when the message carries a
   Chat-Group-Name-Timestamp (or Chat-Group-Name-Changed) header.
   With neither present, the QR-derived name is never overwritten.

Fix

• create_out_broadcast_ex() now stamps Param::GroupNameTimestamp at
  channel creation, so every outgoing channel message advertises the
  name with a timestamp and joiners can reconcile a stale name.
• Migration 153 backfills Param::GroupNameTimestamp on existing
  outgoing broadcast channels.

This keeps the "broadcast channels are always promoted" invariant
intact (Param::Unpromoted is untouched).

Testing

• New regression test test_secure_join_broadcast_reconciles_stale_name
  joins a channel with a manipulated (stale) b= parameter and asserts
  the name is reconciled after a regular channel post. It fails without
  the chat.rs change and passes with it.
• All existing broadcast tests pass, incl. test_broadcasts_name_and_avatar
  (which asserts channels are always promoted) and test_broadcast_change_name.
• cargo fmt clean.

Disclosure

As my Rust knowledge is weak, I have created this patch with help of
Claude Code. Please take this under consideration when reviewing.

[bolstad]

This is a fix for #8250