Add email sync service for automated job application tracking

Background poller fetches emails via IMAP or Microsoft Graph API,
classifies them with Claude Haiku, and creates/updates JobApplication
records automatically. Includes manual sync endpoint and OAuth callback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/go.mod

@@ -5,6 +5,7 @@ go 1.25
 require (
 	github.com/99designs/gqlgen v0.17.88
 	github.com/anthropics/anthropic-sdk-go v1.26.0
+	github.com/emersion/go-imap/v2 v2.0.0-beta.8
 	github.com/gin-gonic/gin v1.11.0
 	github.com/golang-jwt/jwt/v5 v5.3.0
 	github.com/gorilla/websocket v1.5.3
@@ -22,6 +23,8 @@ require (
 	github.com/bytedance/sonic v1.14.0 // indirect
 	github.com/bytedance/sonic/loader v0.3.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
+	github.com/emersion/go-message v0.18.2 // indirect
+	github.com/emersion/go-sasl v0.0.0-20241020182733-b788ff22d5a6 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.8 // indirect
 	github.com/gin-contrib/sse v1.1.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect