feat(feed): cursor pagination for the inbox — scroll-end loads older conversations

[thomaspblock]

What

Cursor-based pagination for the Inbox, stacked on #5834 (per-conversation feed windowing). Scrolling to the end of the inbox list now loads the next page of older conversations instead of stopping at the newest window.

Why

#5834 fixed one chatty thread starving the inbox, but capped the inbox at the newest N conversations — anything older was unreachable. Follow-up agreed with @Thomasp: an inclusive until cursor that pages by conversation.

How

Cursor semantics (the load-bearing decision): the cursor bounds conv_latest — each conversation's newest activity — not event time. Pages are whole conversations at-or-older than the cursor. An event-time cursor would skip every conversation whose latest activity falls between the oldest event in hand and that conversation's conv_latest. The boundary conversation intentionally reappears on the next page (inclusive nostr until) and the client dedupes by event id.

• Relay/DB (buzz-db/src/feed.rs): build_mentions_query gains until on conv_latest; the candidate CTE stays unbounded by the cursor so page membership is stable. Flat feeds (needs_action, activity) bound event time directly. Threaded through all routed Db wrappers and the bridge feed path (filter.until).
• Desktop: get_feed accepts until; getOldestConversationActivity derives the cursor as min(conv_latest) across conversations in hand, mirroring the relay's conversation key (dm:<channel> / NIP-10 root — gated on channel list load for DM keys); useHomeFeedPagination layers fetched-once older pages under the live 30s-polled first page; InboxListPane fetches within one viewport of scroll-end with a tail spinner.
• e2e: bridge mirrors until; new inbox-pagination-screenshots.spec.ts proves scroll-end paging against a live isolated relay; the windowing spec's "after" test now step-scrolls the virtualized list (rows outside the viewport aren't in the DOM) and tolerates sibling-spec conversations.

Pagination depth is bounded by FEED_WINDOW_SCAN_CAP (2000 events) — documented, acceptable.

Proof (Playwright, isolated relay on :3030)

60 single-message conversations seeded; page 1 holds the newest 50 — the oldest markers are beyond it. Scroll-end pages them in.

Before scrolling — first page only (marker 0 absent, asserted):

After scroll-end pagination — oldest conversation (paged mention 0) loaded:

Verification

• buzz-db feed suite 31/31 against Postgres, incl. new paging test (9 conversations, limit 3, cursor walk with inclusive boundary) and a SQL-shape test asserting the cursor doesn't leak into the candidate scan.
• buzz-relay lib suite: 870 passed; 9 failures pre-existing on base 4d74e8e (api::media/admin/telemetry — verified via stash).
• desktop check + typecheck + tests 4778/4778; tauri tests 2415.
• All 3 inbox Playwright integration tests green together against the live relay.

Deliberately out of scope (unchanged from #5834): needs-action promotion for open PRs, participated-threads recall.