Retroactive Unlocks

Backfilling achievement progress from historical data when a new achievement ships.

Why not: Most triggers are events, not state — "save 50 times" or "50 short comments" have no historical record to replay (messages live in Table Storage without per-user mutation counts, saves overwrite a blob). A partial backfill (only state-derived achievements) would be inconsistent and confusing. Definitions count from the moment they ship; everyone races from the same line.