feat(04): Parent conversation review

[tpaulshippy]

Roadmap 04: Parent Conversation Review

Implements #56.

What changed

• Backend: GET /api/activity/chats/ (annotated, paginated, filterable), GET /api/activity/chats/{id}/ (full transcript), GET /api/activity/summary/, send_activity_digests command, device notify flags
• Frontend: PIN-gated Activity inbox with profile chips, safety filter, read-only transcript viewer, notification settings with digest toggle
• Tests: 107 backend (25 new), 59 frontend, e2e walkthrough

Evidence

https://github.com/tpaulshippy/bots/raw/feature/roadmap-04-parent-activity/evidence/pr46-activity.mp4

[gitguardian]

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