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

@@ -3,6 +3,7 @@ using System.Text.Json;
 using JobTrackerApi.Data;
 using JobTrackerApi.Models;
 using JobTrackerApi.Services;
+using JobTrackerApi.Services.EmailProviders;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.EntityFrameworkCore;
@@ -18,15 +19,24 @@ public sealed class GmailController : ControllerBase
     private readonly IGmailJobMatchingService _matching;
     private readonly JobTrackerContext _db;
     private readonly IConfiguration _cfg;
+    private readonly IEmailProviderRegistry _providers;
 
-    public GmailController(IGmailOAuthService gmail, IGmailJobMatchingService matching, JobTrackerContext db, IConfiguration cfg)
+    public GmailController(IGmailOAuthService gmail, IGmailJobMatchingService matching, JobTrackerContext db, IConfiguration cfg, IEmailProviderRegistry? providers = null)
     {
         _gmail = gmail;
         _matching = matching;
         _db = db;
         _cfg = cfg;
+        // Fall back to a single-Gmail registry so direct construction (tests) keeps working.
+        _providers = providers ?? new EmailProviderRegistry(new IEmailProvider[] { new GmailProvider(gmail) });
     }
 
+    // The email provider backing this controller's read paths (search + thread listing),
+    // via the provider-neutral seam. OAuth and Gmail-specific candidate ranking still use
+    // IGmailOAuthService directly until they are generalised.
+    private IEmailProvider Email => _providers.Get("gmail")
+        ?? throw new InvalidOperationException("Gmail email provider is not registered.");
+
     public sealed record GmailImportResultDto(int Imported, int Skipped, string? ThreadId);
     public sealed record GmailImportMessageResultDto(int Imported, int Skipped, string MessageId, string? ThreadId, Correspondence? Message);
     public sealed record ImportGmailMessageRequest(int JobApplicationId, string MessageId);
@@ -383,7 +393,7 @@ public sealed class GmailController : ControllerBase
                 .FirstOrDefaultAsync(x => x.Id == request.JobApplicationId.Value, cancellationToken);
             if (job is null) return NotFound("Job application not found.");
 
-            var threadMessages = await _gmail.ListThreadMessagesAsync(ownerUserId, request.ThreadId.Trim(), cancellationToken);
+            var threadMessages = await Email.ListThreadMessagesAsync(ownerUserId, request.ThreadId.Trim(), cancellationToken);
             var distinctMessageIds = threadMessages
                 .Where(message => !string.IsNullOrWhiteSpace(message.Id))
                 .Select(message => message.Id)
@@ -639,7 +649,7 @@ public sealed class GmailController : ControllerBase
         _db.JobApplications.Add(job);
         await _db.SaveChangesAsync(cancellationToken);
 
-        var threadMessages = await _gmail.ListThreadMessagesAsync(ownerUserId, request.ThreadId.Trim(), cancellationToken);
+        var threadMessages = await Email.ListThreadMessagesAsync(ownerUserId, request.ThreadId.Trim(), cancellationToken);
         var distinctMessageIds = threadMessages.Select(message => message.Id).Where(static id => !string.IsNullOrWhiteSpace(id)).Distinct(StringComparer.Ordinal).ToList();
         // Batch the "already imported?" check with a single query instead of one
         // AnyAsync per message (N+1), mirroring RelinkThread below.
@@ -695,7 +705,7 @@ public sealed class GmailController : ControllerBase
             }
         }
 
-        var threadMessages = await _gmail.ListThreadMessagesAsync(ownerUserId, threadId, cancellationToken);
+        var threadMessages = await Email.ListThreadMessagesAsync(ownerUserId, threadId, cancellationToken);
         var distinctMessageIds = threadMessages.Select(message => message.Id).Where(static id => !string.IsNullOrWhiteSpace(id)).Distinct(StringComparer.Ordinal).ToList();
         var existingMessageIds = await _db.Correspondences
             .Where(message => message.JobApplicationId == job.Id && message.ExternalMessageId != null && distinctMessageIds.Contains(message.ExternalMessageId))
@@ -793,7 +803,7 @@ public sealed class GmailController : ControllerBase
     public async Task<IActionResult> Messages([FromQuery] string? query, [FromQuery] int maxResults = 12, CancellationToken cancellationToken = default)
     {
         var ownerUserId = GetRequiredOwnerUserId();
-        var items = await _gmail.ListMessagesAsync(ownerUserId, query, maxResults, cancellationToken);
+        var items = await Email.SearchAsync(ownerUserId, query, maxResults, cancellationToken);
         return Ok(items);
     }
 
@@ -897,7 +907,7 @@ public sealed class GmailController : ControllerBase
 
             foreach (var threadId in linkedThreadIds)
             {
-                var threadMessages = await _gmail.ListThreadMessagesAsync(ownerUserId, threadId, cancellationToken);
+                var threadMessages = await Email.ListThreadMessagesAsync(ownerUserId, threadId, cancellationToken);
                 var distinctThreadMessages = threadMessages
                     .Where(message => !string.IsNullOrWhiteSpace(message.Id))
                     .GroupBy(message => message.Id, StringComparer.Ordinal)