fix(categories): correct all seven smart-folder bugs at the source (PHASE 2)

Root causes + fixes:
- Archive leaked Sent mail: filter was just 'not in inbox'. Added ExcludeGmailLabels;
  Archive now excludes SENT/DRAFT/SPAM/TRASH/CHAT by label.
- Read Later / Pinned / Unlabelled returned EVERYTHING (no folder case -> default all).
  Pinned = IsImportant; Read Later = new local IsReadLater marker (+toggle endpoints);
  Unlabelled = HasUserLabels=false.
- Trash & Spam always empty: IncludeSpamTrash=false + sync never set IsTrashed nor created
  EmailLabel rows. Now fetches spam/trash, sets IsTrashed from the TRASH label, and links
  every message to its Gmail labels (also fixes Sent/Spam/category-by-label/Unlabelled).
- Old Mail: now strictly older than 6 months (was 1 year).

Guardrail: sync upsert is now idempotent (drops prior copy before reinsert) so re-sync
can't duplicate a message or leave stale labels. 4 new filter tests; 65 backend + frontend
green; migration is a single non-destructive column add.

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

src/InboxIntel.Domain/Entities/Email.cs

@@ -40,6 +40,10 @@ public class Email : AuditableEntity
     public bool IsTrashed { get; set; }
     public bool HasAttachments { get; set; }
 
+    /// <summary>Local "Read Later" marker (not a Gmail concept) — toggled by the user so the
+    /// Read Later smart folder shows only explicitly flagged mail, never everything.</summary>
+    public bool IsReadLater { get; set; }
+
     // Unsubscribe signals captured at parse time.
     public bool HasListUnsubscribe { get; set; }
     public string? ListUnsubscribeRaw { get; set; }