feat(web-api): gate bouncer creation by plan

Block free-tier users from creating dedicated bouncers with a 403
response. Show shared bouncer UI with chat.irc.now link and upgrade CTA
for free users on the dashboard. Add migrate checkbox to the bouncer
create form for data migration from shared to dedicated bouncer.

Signed-off-by: Josephine Pfeiffer <hi@josie.lol>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Josephine Pfeiffer <hi@josie.lol>