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.Api/Controllers/EmailController.cs

@@ -72,6 +72,22 @@ public class EmailController : ApiControllerBase
     [HttpPost("{id:guid}/untrash")]
     public Task<IActionResult> Untrash(Guid id, CancellationToken ct) => Act(id, CleanupActionType.Archive, ct);
 
+    /// <summary>Toggle the local-only Read Later marker (no Gmail side effect).</summary>
+    [HttpPost("{id:guid}/readlater")]
+    public Task<IActionResult> ReadLater(Guid id, CancellationToken ct) => SetReadLater(id, true, ct);
+
+    [HttpPost("{id:guid}/unreadlater")]
+    public Task<IActionResult> UnreadLater(Guid id, CancellationToken ct) => SetReadLater(id, false, ct);
+
+    private async Task<IActionResult> SetReadLater(Guid id, bool value, CancellationToken ct)
+    {
+        var email = await _db.Emails.FirstOrDefaultAsync(e => e.Id == id && e.UserId == UserId, ct);
+        if (email is null) return NotFound();
+        email.IsReadLater = value;
+        await _db.SaveChangesAsync(ct);
+        return Ok();
+    }
+
     /// <summary>
     /// Inline unsubscribe. Detects the unsubscribe mechanism for the email's sender,
     /// then executes it (HTTP one-click or HTTP link). mailto targets cannot be sent