ENH: Add project validation metadata

[GibranAlfa]

Resolves #58

Added project config validation metadata so API can record when saved project data was last checked against external/project sources.

This adds:

• ValidationRecord, with last_validated_at and last_validated_by
• ProjectValidation, with masterdata_smda and rms_project
• ProjectConfig.validation, defaulting to empty validation metadata

Example serialized shape:

{
  "validation": {
    "masterdata_smda": {
      "last_validated_at": "2026-06-25T10:30:00Z",
      "last_validated_by": "muga"
    },
    "rms_project": {
      "last_validated_at": "2026-06-25T10:31:00Z",
      "last_validated_by": "muga"
    }
  }
}

Empty/default shape:

{
  "validation": {
    "masterdata_smda": null,
    "rms_project": null
  }
}

The API would fill them like this:

record = ValidationRecord(
    last_validated_at=datetime.now(UTC),
    validated_by=getpass.getuser(),
)

self._fmu_dir.set_config_value(
    "validation.masterdata_smda",
    record.model_dump(),
)

Checklist

• Tests added (if not, comment why) - just model update
• 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 ✅

[slangeveld]

👍

[slangeveld]

This should probably be "User who performed the last validation" to be alligned with docstring for last_validated_at