DECISIONS:

• History loading is cross-conversation, scoped to sender_key (Identity Sync resolution intentionally NOT applied — user sees the messages bound to this web sender only; WhatsApp-mirror handling stays as is).
• Each click loads up to 50 messages older than the oldest currently-visible message; button disappears when fewer than 50 returned.
• Date dividers computed client-side from message timestamps using the WhatsApp convention: Today / Yesterday / (within 7 days) /
  .
• Bubble style: green (#dcf8c6 light or #005c4b dark) for user, white/dark-grey for Carol; timestamp inside bubble bottom-right; subtle WhatsApp-like background.
• Stranger mode separation preserved: history endpoint scopes by the same sender_key the request comes in with.

REQUIREMENTS:

• GET /api/conversations/{conv_id}/history?before=&limit=50 returns prior messages for the same sender_key.
• Frontend button renders only when more messages may exist; loading preserves scroll anchor.
• Date dividers correctly group messages and update when new messages arrive.
• No regression on the existing send + receive + upload flow.

SUCCESS CRITERIA:

• After load, prior-day messages appear above today's with correct dividers.
• Button hides when older messages exhausted.
• WhatsApp-style colors + timestamp + dividers visible on the live URL.
• Existing tests still pass.

STRATEGY:

• Backend: small SELECT on chat.db joined across conversations by sender_key, ordered DESC by id, limited.
• Frontend: scroll-anchor preserve trick (capture scrollHeight pre-prepend, restore post-prepend), divider renderer, restyled CSS.

PLAN STEPS: 1. Add /history endpoint + sender-key-scoped query. 2. Restyle bubbles + background CSS to WhatsApp aesthetic. 3. Implement date-divider rendering on initial load + on prepend + on new message arrival. 4. Add Load Previous button + handler with scroll preservation. 5. Browser smoke test on live URL. 6. Update test_ini459_uploads or add a new test for the history endpoint.

BUDGET: $2.50