feature request: add sync to fanfou feature

[shukebeta]

After user allowing fanfou login, we can setup what notes can be synced to fanfou at the front end.
We need to support:

• use case 1: sync all public notes
• use case 2: sync a note if it has specific tag

[shukebeta]

Planner refinement — blocked for product/scope decision

This is a net-new feature — grep -rin fanfou across the repo is clean, so there is no existing scaffolding to extend. The two use cases ("sync all public notes", "sync if specific tag") require decisions that are yours, not the planner's:

Open questions for shuke:

1. Fanfou API availability & shape. Fanfou's write-API status is unclear. Is there a documented, working OAuth + status-post API to integrate against? (Determines feasibility.) For contrast: Mastodon is per-user-instance, Telegram is bot+channel — what is the Fanfou auth/account model?
2. Auth/account linkage. "After user allowing fanfou login" implies a new external-account linkage (OAuth) and a per-user credential store, paralleling the Mastodon account integration. Confirm that's the intended auth path and where credentials live.
3. v1 scope. Both use cases, or MVP with one? "All public notes" is a bulk backfill + ongoing sync; "tag-based" needs a tag→sync-target config model + UX. These are different sizes — which first?
4. Sync mechanics. Realtime via the existing Redis SyncQueue (mirroring the Telegram/Mastodon handlers), or batch? Edit/delete fan-out? Backfill of historical public notes?

Leaving this blocked until you weigh in. Once API + scope are decided it becomes a straightforward planner job against the Mastodon/Telegram integration template (handler + ISyncNoteService + SyncQueue registration + user-account settings).

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Audit: class=feature (product/scope). Requires Fanfou API feasibility + auth model + v1 scope decisions that are shuke's to make.