feat: AI categorisation fallback, unsubscribe confidence scoring, one-line summaries

Heuristic classifier falls back to AI only when it can't determine a category;
unsubscribe confidence blends method reliability with an AI safety opinion and
surfaces it in the Unsubscribe Manager; emails get an on-demand AI one-line
summary in the detail pane. All AI calls degrade gracefully when AI is disabled
or the provider errors.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/InboxIntel.Infrastructure/Sync/SyncService.cs

@@ -24,14 +24,16 @@ public class SyncService : ISyncService
     private readonly ILogger<SyncService> _logger;
     private readonly GmailSyncOptions _options;
     private readonly ISyncQueue _queue;
+    private readonly IAiService _ai;
 
-    public SyncService(AppDbContext db, IGmailService gmail, ILogger<SyncService> logger, IOptions<GmailSyncOptions> options, ISyncQueue queue)
+    public SyncService(AppDbContext db, IGmailService gmail, ILogger<SyncService> logger, IOptions<GmailSyncOptions> options, ISyncQueue queue, IAiService ai)
     {
         _db = db;
         _gmail = gmail;
         _logger = logger;
         _options = options.Value;
         _queue = queue;
+        _ai = ai;
     }
 
     public async Task QueueSyncAsync(Guid userId, bool fullSync, CancellationToken ct = default)
@@ -232,6 +234,12 @@ public class SyncService : ISyncService
         var sender = await ResolveSenderAsync(userId, d.FromAddress, d.FromDisplayName, ct);
         var thread = await ResolveThreadAsync(userId, d.GmailThreadId, d.Subject, d.Snippet, d.SentAtUtc, ct);
 
+        var category = HeuristicClassifier.Classify(d);
+        // The heuristic rule set falls back to Personal when nothing more specific
+        // matches; if AI is enabled, give it a shot at a better category.
+        if (category == EmailCategory.Personal && _ai.IsEnabled)
+            category = await _ai.ClassifyFallbackAsync(d.Subject, d.FromAddress, d.Snippet, ct);
+
         var email = new Email
         {
             UserId = userId,
@@ -252,7 +260,7 @@ public class SyncService : ISyncService
             HasListUnsubscribe = d.HasListUnsubscribe,
             ListUnsubscribeRaw = Trunc(d.ListUnsubscribeRaw, 2048),
             SupportsOneClickUnsubscribe = d.SupportsOneClickUnsubscribe,
-            Category = HeuristicClassifier.Classify(d)
+            Category = category
         };
         _db.Emails.Add(email);