fix(auth): send signers' directory email, never the sign-in placeholder

[lorenzocorallo]

Summary

• /api/internal/esign-users returned the raw user.email, which for PoliNetwork (Entra) and Telegram sign-ins is the placeholder <provider>.<id>@identity.invalid. The signing picker showed it, and backend#79 saved it as the address signature requests are emailed to.
• It now returns the address administrators see in the user directory (contactEmails: the linked @polinetwork.org address first), or null.
• contactEmails gains an everyone scope that reads all accounts in one query, without one parameter per user.

Verification

• tsc --noEmit, vp test (305 passed), vp check, pnpm run build.
• The HTTP integration test (updated to expect email: null for a placeholder) needs the integration database and was skipped.

Rollout

Deploy with or before PoliNetworkOrg/backend's matching fix; no contract change.

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📥 Commits

Reviewing files that changed from the base of the PR and between f013796 and 9d03d03.

📒 Files selected for processing: 3

• src/auth/access-snapshot.integration.test.ts
• src/auth/contact-email.ts
• src/routes/api/internal/esign-users.ts

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