First slice toward multi-provider email (Gmail + Microsoft Graph + IMAP +
manual/free-text, per docs/remaster/PRODUCT_DIRECTION.md). Adds a provider-
neutral contract (search / list-thread / get-message / get-connection) with
neutral DTOs, a registry to resolve providers by key, and a GmailProvider that
adapts the existing IGmailOAuthService to it.
No behaviour change: the seam is registered in DI but not yet consumed. Follow-up
slices migrate GmailController's read paths onto IEmailProvider (folding in the
N+1 fixes) and add MicrosoftGraphProvider / ImapProvider / a manual provider.
Build clean; backend suite 135/135 green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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