docs(api): clarify synced_at semantics (1970-01-01 sentinel)

[MikaKerman]

Summary

Adds a note to the Incremental sync guide clarifying what synced_at means, after smoke-testing the beta surfaced synced_at: "1970-01-01T00:00:00" on a subset of columns.

• synced_at marks created / last-changed, not last-seen.
• Unchanged objects keep their existing value; some pre-existing rows carry a 1970-01-01 epoch placeholder.
• This is safe for incremental: every object is returned by the initial full scan regardless of synced_at, and any later change re-emits it in the synced_since feed.
• Guidance: treat synced_at as a change marker for upserts, not an absolute freshness signal.

Addresses APP-1550. The matching OpenAPI field descriptions (synced_at on assets/columns) are sharpened in elementary-internal (spec source of truth) and reach the published snapshot via the cross-repo sync (APP-1546).

Test plan

• mint dev renders /api/incremental-sync (HTTP 200) with the new note.
• Mintlify validation passes.

Made with Cursor

[linear]

APP-1550

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