chore(db): automate master.xml append in db:draft-changelog script

[FelixTJDietrich]

Context: #1290 (epic #1097 server foundations) deliberately retained master.xml with explicit <include> per changelog rather than switching to <includeAll/>. Rationale in ADR 0006: two parallel migration PRs produce a natural merge conflict in master.xml that forces authors to agree on order; <includeAll/> would silently interleave them lexicographically.

The trade-off is a manual step: drafting a new changelog today requires both running the db:draft-changelog script AND remembering to append the new file to master.xml. Easy to forget.

What needs to ship

Update db:draft-changelog (the script that generates a new timestamped changeset under server/src/main/resources/db/changelog/) so that it also appends an <include file="changelog/<timestamp>_changelog.xml" relativeToChangelogFile="true"/> line to master.xml immediately before the closing </databaseChangeLog>.

The two-PR ordering conflict surface is preserved (parallel PRs still conflict on the new line; authors still have to resolve it deliberately). The forget-to-edit-master footgun goes away.

Acceptance

• db:draft-changelog updates master.xml automatically
• docs/contributor/database-migration.mdx updated to remove the manual append step
• Test that running the script twice in quick succession produces two correctly-ordered <include> lines
• CI gate: a changelog file exists under db/changelog/ without a corresponding <include> in master.xml fails the build