refactor: rename the webinar page to meeting

[dasniko]

Related to #16 — the feature is in use (the handbuch documents it and JUG DA relies on it), but "webinar" describes a format nobody here actually runs. These are online meetings people dial into, and the page has said "Meeting" in its own headline all along.

Draft: the rename is complete and green, but it changes a public URL and stored data, so it wants a look before it goes in.

What changes

Layer	Before	After
URL	/webinar/{tenant}/{eventId}	/meeting/{tenant}/{eventId}
Resource	WebinarResource	MeetingResource
Templates	templates/webinar/webinar.html	templates/meeting/meeting.html
Event data key	webinarLink	meetingLink
Content key	webinar.tools	meeting.tools
German UI	"Webinar-Link", "Webinar-Seite"	"Meeting-Link", "Meeting-Seite"

The old URL is gone, not redirected — as agreed, since the only places handing it out are the calendar entry, the bulk-mail draft and the admin button, and all three now emit the new one. Links in already-sent mails and in .ics files participants imported will 404.

V9__rename_webinar_to_meeting.sql renames both stored keys. V4 and V8 keep the old name on purpose: Flyway checksums a migration once it has run, so editing one makes a running instance refuse to start. They are the only files left containing the word.

A bug the rename exposed

b2d544a is a fix, not a rename: the page answered 500 whenever no link was stored yet. Qute renders strictly and {eventData.meetingLink} had no key to resolve. That is exactly the window that matters — the page goes up on the day of the event, the orga team pastes the link in sometimes minutes before the start, and participants refreshing in between got an error page. handbuch.adoc:885 has always described this case as "nicht verfügbar"; now the page delivers that.

It stayed invisible because the only test hitting the page ran after a test that happened to upload a link. JUnit orders methods by a hash of their names, so renaming one reordered the class and the 500 fell out. That test now stores its own link, and testMeetingPageSaysNotAvailableWhileNoLinkIsStored covers the empty case — verified by removing the guard and watching it fail.

Verification

Full suite on Temurin 25.0.2: 47 tests green, including the fresh database built from V1…V9.

Review notes

• Commits are layered: code rename, stored keys + migration, the 500 fix, docs.
• The seeded German content values still say "Zoom-Meeting" etc. — those are tenant-owned texts, untouched by the key rename.
• If any JUG website links the old page directly, it needs updating; the iframe embeds point at /registration/... and are unaffected.