feat: metrics dashboards with SVG charts, CSV export, leaderboard (phase 10)

- aggregations over the immutable bountyAwards ledger: totals, weekly
  buckets ($dateTrunc), per-developer and per-customer groupings
- timeline-derived: approval rate, time logged, assigned→approved lead
  time, imported→published atomization lead time, open board depth
- developer + consultant dashboards (admin = global consultant view),
  date-range filters, CSV export of the ledger
- leaderboard (top 10 by bounty) honoring the new leaderboardOptOut
  profile setting
- hand-rolled SVG line and bar charts (~150 lines, no chart library)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/domain/user.go

@@ -53,8 +53,9 @@ type NotificationPrefs struct {
 }
 
 type UserSettings struct {
-	Theme         string            `bson:"theme" json:"theme"`
-	Notifications NotificationPrefs `bson:"notifications" json:"notifications"`
+	Theme             string            `bson:"theme" json:"theme"`
+	Notifications     NotificationPrefs `bson:"notifications" json:"notifications"`
+	LeaderboardOptOut bool              `bson:"leaderboardOptOut" json:"leaderboardOptOut"`
 }
 
 type User struct {