Add custom metadata to runs and units

Tag runs and units with your own typed key/value fields so traceability data, and qualification flags live next to the test data they describe.

• Added a metadata editor on run and unit detail pages with type-aware icons and inline edit
• Surfaced metadata in the V2 API as a plain {key: value} dict; types (string, number, bool) are detected automatically
• Added metadata filters on run and unit list endpoints (in, contains, gte, lte, eq)
• Tracked every metadata change in a per-entity activity timeline