[FEATURE] Client-activity notifications (gallery opened / favorited / downloaded)

[Luca-Timo]

Notify the photographer on "gallery opened / client favorited / downloaded." PicPeak has email + webhooks but no unified activity-notification surface.

• Notify on gallery open / favorite / download

Effort: S — mostly wiring existing events into a notification surface. Good first candidate.

Parked idea — no timeline.