Exclude Gmail send-as aliases from received-message participants

[atomantic]

Summary

Fixes a Tribe-outreach gap where a genuinely unanswered 1:1 email delivered to one of the owner's Gmail send-as aliases (rather than the primary account.email) was never surfaced as an outreach nudge.

In messageActivityCandidates, a received message's participants were computed as [from, ...to, ...cc] filtered to drop only account.email. When a 1:1 message was delivered to a send-as alias, the alias survived the filter, so the message had two counterparts (sender + owner alias) and findUnansweredTribeThreads treated it as a group thread and skipped it. The prior #2796 alias handling only covered the SENT direction (via Gmail's SENT label); the received-to-alias case was unhandled, and the account stored only a single email to filter against.

Changes

• Fetch & persist aliases — messageGmailSync.js calls gmail.users.settings.sendAs.list on each sync (best-effort; a null sentinel on failure keeps the stored set rather than clobbering it). Persisted onto the account as sendAsAliases (lowercased, deduped) via a new updateSendAsAliases in messageAccounts.js.
• Filter the whole owner set — messageActivityCandidates now builds an owner-address set of { selfEmail, ...sendAsAliases } and excludes all of them from received-message participants. The existing SENT-label direction logic is unchanged.
• Back-compat — absent sendAsAliases degrades to today's behavior (filter on primary email only); missing/failed alias fetch never crashes and produces no false person records. Additive account field, no migration needed.
• Zod schema (routes/messages.js) extended with the additive sendAsAliases array for parity.

Follow-up

• Backfill existing Gmail activity rows when send-as aliases are first learned (follow-up to #2831) #2855 — backfill existing Gmail activity rows synced before aliases were learned (the ON CONFLICT DO NOTHING insert contract leaves already-stored rows with stale participants until they age out of the 14-day window; split out to avoid changing the global dedup semantics). Surfaced by codex review.

Test plan

• server/services/humanActivity.test.js — new cases: a received 1:1 delivered to an alias stays a single-counterpart 1:1; back-compat (no aliases stored → only primary email filtered).
• server/services/messageGmailSync.test.js — new fetchSendAsAliases cases: lowercased aliases returned, empty array vs null sentinel on API failure.
• Full server suite green (humanActivity, messageGmailSync, messageSync, messageAccounts, routes/messages — 149 tests).

Closes #2831