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/Migrations/AppDbContextModelSnapshot.cs

@@ -467,6 +467,9 @@ namespace InboxIntel.Infrastructure.Migrations
                         .ValueGeneratedOnAdd()
                         .HasColumnType("uuid");
 
+                    b.Property<double>("Confidence")
+                        .HasColumnType("double precision");
+
                     b.Property<DateTimeOffset>("CreatedAtUtc")
                         .HasColumnType("timestamp with time zone");