chat history pagination: paged load instead of full-session fetch ...

Backend:
- PgSessionStore.get_meta — light session head (sessions row + COUNT),
  no message payload
- PgSessionStore.get_history_page — page of display history (archive ∪
  hot UNION), newest-first page, oldest-first result, limit+1 peek for
  has_more; each message stamped metadata.display_index (ROW_NUMBER-1
  over the full display history) so client ids stay stable across
  paged loads; dangling-tool-call repair keeps real ranks by
  sequence-number lookup
- GET /sessions/{id}/meta and GET /sessions/{id}/messages/history
  (before_seq cursor, limit ≤200)

Webclient:
- loadSession/reloadSession fetch meta + newest 200-item page
  (Promise.all), archive state from the page itself
- buildMessageList ids (h_*) and rawIndices use the global
  display_index — feedback keys and search-jump stay stable when
  history is paged
- scroll-up loads older pages through the same history endpoint
  (archive table alone misses sessions whose threshold never moved)
- search-jump pulls pages until the target index is covered (bounded)

Tests: store unit tests (ranks, has_more peek, placeholder shift) +
vitest updates; full pytest 1231 passed, vitest 94 passed.