Restore server-side message pagination on HTTP endpoint

[Jasonnnz]

Summary

Restores server-side message pagination that was lost during the WebSocket→HTTP transport migration (#16207). The daemon's GET /v1/messages endpoint now honors beforeTimestamp for cursor-based pagination, enabling the client to load older messages when scrolling up. Uses Option A: only paginate when beforeTimestamp is present, so initial load and reconnect behavior is unchanged (zero regression risk).

Changes

• M1 (M1: Add composite DB index and getMessagesPaginated() #21562): Added composite DB index (conversation_id, created_at) and getMessagesPaginated() / getLastAssistantTimestampBefore() query functions
• M2 (M2: Wire pagination into handleListMessages HTTP endpoint #21569): Wired pagination into handleListMessages — parses beforeTimestamp/limit, returns hasMore/oldestTimestamp/oldestMessageId, seeds interface diffs correctly
• M3 (M3: Client-side safety fixes for pagination edge cases #21565): Client-side safety fixes — removed trimOldMessagesIfNeeded() from pagination branch (prevents infinite fetch-trim loop), added 60s timeout to clear stuck loading state
• M4 (M4: Add pagination test suite for handleListMessages #21578): Added 9-case pagination test suite covering backward compat, cursor behavior, strict exclusivity, metadata, validation

Milestone PRs (merged into feature branch)

• M1: Add composite DB index and getMessagesPaginated() #21562: M1 — Add composite DB index and getMessagesPaginated()
• M2: Wire pagination into handleListMessages HTTP endpoint #21569: M2 — Wire pagination into handleListMessages HTTP endpoint
• M3: Client-side safety fixes for pagination edge cases #21565: M3 — Client-side safety fixes for pagination edge cases
• M4: Add pagination test suite for handleListMessages #21578: M4 — Add pagination test suite for handleListMessages

Project issue

Closes #21550

Test plan

• cd assistant && bun test src/__tests__/list-messages-attachments.test.ts — all 17 tests pass
• Open a conversation with 100+ messages, scroll up — verify older messages load progressively
• Verify scroll position stays stable during page loads
• Open a short conversation (<50 messages) — verify no pagination sentinel appears
• Kill daemon mid-pagination — verify spinner clears after ~60s

---

[Jasonnnz]

@codex review

[Jasonnnz]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 53aa620718

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Include message-id tie-breaker in pagination filter

The pagination query filters only on created_at < beforeTimestamp, so if multiple messages share the same millisecond as the page boundary, the next page permanently skips the remaining same-timestamp messages. This is a real data-loss-in-pagination case for legacy/imported conversations where duplicate millisecond timestamps exist, and the commit already exposes oldestMessageId but never uses it to disambiguate cursors. Please paginate with a stable (created_at, id) cursor (and request-side beforeMessageId) to avoid dropped history rows.

Useful? React with 👍 / 👎.