Add a friends and follows activity feed

[ALeonard9]

Story

As a druthers user, I want to see what my friends and the people I follow have been ranking and adding, so that druthers gives me a reason to come back beyond my own lists.

Acceptance Criteria

• A feed shows recent activity from accepted friends and followed users
• Events include ranking a title and adding one to a watchlist, at minimum
• Activity is only shown for shelves the viewer is actually permitted to see, evaluated per event against the owner's current tiers
• Retroactive privacy holds: lowering a shelf's tier removes its past events from other people's feeds
• The feed is paged and bounded, and does not degrade as the graph grows
• Users can opt out of contributing to other people's feeds

Context

Part of #272. Explicitly the "down the road" item — filed so it is not lost, not scheduled with the rest of the epic.

Adam: "Down the road I'd want the ability to see friends / follows activity (movie ranked, watchlist add)."

Depends on #275 and #276 for the graph, and #277 for the tier rules. router_activity.py already models per-user activity and is the natural foundation.

Estimate

• Recommended model: Opus 4.8 — fan-out and per-event authorization that has to stay correct as tiers change over time.
• Human effort: L — needs product decisions on feed composition, ranking, and opt-out before it can start.

Notes for Implementation

The retroactive-privacy requirement is the hard part and rules out denormalizing visibility onto activity rows at write time. Evaluate against current tiers at read time, or invalidate on tier change.