feat(coexist): keep synced history out of AI context by default

[viethung0o0]

Summary

Adds an AI-context boundary for coexist history sync on WhatsApp, Messenger, and Instagram.

• By default, messages imported by a coexist sync are excluded from AI context: after each sync batch, Conversation.aiContextLastMessageId advances to the newest sync-inserted message (either direction), so AI features (AI agent, automated responses, flow AI steps) only consider newer messages.
• The coexist onboarding dialog gains a switch (default off) — "AI reads synced history" — letting a workspace opt in to feeding the imported history to the AI instead.
• New coexistAiReadsSyncedHistory column on the three integration tables (additive migration, default false); the flag is written only on enable, never on decline/disable.
• Conversation activity + AI-marker updates are batched into a single statement per sync bulk via the business layer, with advance-only semantics, per-workspace scoping, and cache invalidation.

Behavior notes

• Turning the switch off later only stops future marker advancement; it does not re-expose already-skipped history.
• Marker updates are forward-only and idempotent across sync retries; markers set by delete-message-history are never regressed.
• Existing API clients are unaffected: the new request field is optional and defaults to false.

Test plan

• Worker: 1500/1500 (incl. direction-agnostic newest-message cases, fallback-timestamp batches, dedup/advance-only, per-channel flag threading)
• Builder: 1547/1547 (coexist API schema/write paths, popup default + toggle, i18n parity across 20 locales)
• Business: 963/963 (batched conversation update semantics + cache invalidation)
• Typecheck clean (database, business, worker, builder); lint clean

Deployment

Apply the additive migration 20260818153651_coexist-ai-reads-synced-history before starting the new processes. With the default false, existing coexist integrations start excluding synced history from AI context on their next sync run.