feat(instagram): coexist history sync for native Instagram

[viethung0o0]

Summary

Adds a coexist option for the native Instagram channel, matching the existing Messenger and WhatsApp coexist behavior. When enabled after connecting an Instagram account, existing Instagram Direct conversations and recent message history are imported into the inbox while live messages continue to flow.

Scope

• Enable/disable coexist for native Instagram (type: "instagram" only; instagram-facebook is unaffected).
• Background sync imports contacts and recent message history idempotently, with resumable progress and bounded concurrency.
• Coexist state access moved behind a repository + business service; Messenger and WhatsApp coexist behavior is preserved.
• Instagram message history is limited to what Meta exposes (20 most recent messages per conversation); the feature does not promise full history.

Notes

• Includes a database migration (enum value + coexistEnabled column). Apply on deploy.
• Pre-run quota gating for large imports is left as a follow-up product decision.

Test plan

• pnpm lint on changed files
• Type-check: database, worker, builder
• Unit/integration tests: coexist repository, service, Instagram sync, adapters, scheduler dispatch, attachment download
• Messenger + WhatsApp coexist tests remain green (no regression)
• Verify end-to-end after migration is applied in staging