refactor(api): extract Gmail DTOs/parsers, batch N+1 loops

- Move inline DTOs to GmailDtos.cs, pure parse helpers to GmailParsing.cs
- Batch per-message existence checks in CreateSuggestedJob/RefreshLinkedThreads
- Remove redundant second pass in RelinkThread, reuse existing HashSet
- Replace ToListAsync+scan with FirstOrDefaultAsync for GmailReviewDecisions lookups

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

JobTrackerApi/Controllers/GmailController.cs

@@ -7,13 +7,14 @@ using JobTrackerApi.Services.EmailProviders;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.EntityFrameworkCore;
+using static JobTrackerApi.Services.GmailParsing;
 
 namespace JobTrackerApi.Controllers;
 
 [ApiController]
 [Route("api/gmail")]
 [Authorize]
-public sealed partial class GmailController : ControllerBase
+public sealed class GmailController : ControllerBase
 {
     private readonly IGmailOAuthService _gmail;
     private readonly IGmailJobMatchingService _matching;