feat: add email notification service with SMTP support

- Create pkg/email package (sender, templates, types)
- SMTP client with TLS, rate limiting, async sending
- HTML email templates with GooSeek branding
- Integrate welcome + password reset emails in auth-svc
- Add limit warning emails (80%/100%) in llm-svc middleware
- Add space invite endpoint with email notification in thread-svc
- Add GetUserEmail helper in JWT middleware
- Add SMTP config to .env, config.go, K8s configmap

Made-with: Cursor

backend/pkg/middleware/jwt.go

@@ -212,6 +212,14 @@ func GetUserTier(c *fiber.Ctx) string {
 	return user.Tier
 }
 
+func GetUserEmail(c *fiber.Ctx) string {
+	user := GetUser(c)
+	if user == nil {
+		return ""
+	}
+	return user.Email
+}
+
 func RequireAuth() fiber.Handler {
 	return func(c *fiber.Ctx) error {
 		user := GetUser(c)