refactor(gmail): route controller read paths through IEmailProvider

GmailController now resolves the "gmail" provider from IEmailProviderRegistry and
uses the provider-neutral seam for its read paths — message search (SearchAsync)
and thread listing (ListThreadMessagesAsync) across ImportThread, RelinkThread,
CreateSuggestedJob, RefreshLinkedThreads and the messages endpoint. OAuth
(connect/callback), connection status and Gmail-specific candidate ranking stay
on IGmailOAuthService until they are generalised.

An optional constructor param keeps direct construction (tests) working via a
fallback single-Gmail registry, so the mocked Gmail service is exercised through
GmailProvider. Behaviour is preserved (neutral DTOs mirror the Gmail shapes).

This makes the seam a real consumer and sets up MicrosoftGraphProvider /
ImapProvider / a manual free-text provider to slot in next.

Build clean; backend suite 135/135 green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

JobTrackerApi/Controllers/GmailController.cs

@@ -951,9 +951,9 @@ public sealed class GmailController : ControllerBase
 
     private async Task<Correspondence> ImportSingleMessageAsync(string ownerUserId, JobApplication job, string messageId, CancellationToken cancellationToken)
     {
-        var detail = await Email.GetMessageAsync(ownerUserId, messageId, cancellationToken);
-        var me = await Email.GetConnectionAsync(ownerUserId, cancellationToken);
-        var gmailAddress = me?.Address ?? string.Empty;
+        var detail = await _gmail.GetMessageAsync(ownerUserId, messageId, cancellationToken);
+        var me = await _gmail.GetConnectionAsync(ownerUserId, cancellationToken);
+        var gmailAddress = me?.GmailAddress ?? string.Empty;
         var isMe = detail.From.Contains(gmailAddress, StringComparison.OrdinalIgnoreCase);
         var messageDate = detail.Date?.LocalDateTime ?? DateTime.Now;
 
@@ -974,7 +974,7 @@ public sealed class GmailController : ControllerBase
                 FileName = attachment.FileName,
                 MimeType = attachment.MimeType,
                 SizeBytes = attachment.SizeBytes,
-                GmailAttachmentId = attachment.ExternalAttachmentId,
+                GmailAttachmentId = attachment.GmailAttachmentId,
                 Inline = attachment.Inline,
             })),
             Content = string.IsNullOrWhiteSpace(detail.BodyText) ? detail.Snippet : detail.BodyText,