fix session report submission: raise body size limit from 1 MB to 20 MB

Reports with screenshots and agent logs easily exceed 1 MB after base64
encoding. 20 MB accommodates 5 MB agent logs (~6.7 MB base64) plus
multi-MB screenshots plus CSV/TXT exports.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Backend/AutopilotMonitor.Functions/Functions/Reports/SubmitSessionReportFunction.cs

@@ -45,10 +45,11 @@ namespace AutopilotMonitor.Functions.Functions.Reports
                 var tenantId = requestCtx.TenantId;
                 var userIdentifier = requestCtx.UserPrincipalName;
 
-                // Request body size limit (1 MB)
+                // Request body size limit (20 MB — must accommodate base64-encoded agent logs,
+                // screenshots, plus CSV/TXT exports; base64 adds ~33% overhead)
                 if (req.Headers.TryGetValues("Content-Length", out var clValues)
                     && long.TryParse(clValues.FirstOrDefault(), out var contentLength)
-                    && contentLength > 1_048_576)
+                    && contentLength > 20_971_520)
                 {
                     var tooLarge = req.CreateResponse(HttpStatusCode.BadRequest);
                     await tooLarge.WriteAsJsonAsync(new { success = false, message = "Request body too large" });