ENH: Add schema migration support for versioned resources - #298

Resolves #249
Resolves #305

Most lines are documentation and tests.

Add automatic, forward-only schema migrations for project config, user config, and internal mappings.

• Load: Old data is migrated in memory. Loading does not change the stored file, create a cache revision, or add a changelog entry.
• Save: When the resource is next saved, the original JSON is preserved as a normal cache revision before the migrated data is written.
• Cache: Existing cache retention rules apply. Old cache revisions are migrated in memory when read.
• Restore: Old cache revisions are migrated to the current schema before they are restored. Restore keeps the existing changelog behavior.
• User experience: Migration happens automatically. Users do not need a separate endpoint or migration action.
• Added a backup for resources that require migration. The exact original JSON is saved under .fmu/migration-backups/, with timestamp, model name, and source schema version in the filename.. Backup failures are logged but do not block the save. Users can copy a backup manually when rolling back to an older release.
• Changed default cache_max_revisions from 5 to 10.

This also adds maintainer documentation and exports MigrationError from the public package API.

Related issue in fmu-settings-api is made to update the API to catch MigrationError properly.

Checklist

• Tests added (if not, comment why)
• Test coverage equal or up from main (run pytest with --cov=src/ --cov-report term-missing)
• If not squash merging, every commit passes tests
• Appropriate commit prefix and precise commit message used
• All debug prints and unnecessary comments removed
• Docstrings are correct and updated
• Documentation is updated, if necessary
• Latest main rebased/merged into branch
• Added comments on this PR where appropriate to help reviewers
• Moved issue status on project board
• Checked the boxes in this checklist ✅