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)

backend/pkg/middleware/llm_limits.go

@@ -3,14 +3,17 @@ package middleware
 import (
 	"context"
 	"database/sql"
+	"log"
 
 	"github.com/gofiber/fiber/v2"
 	"github.com/gooseek/backend/internal/usage"
+	"github.com/gooseek/backend/pkg/email"
 	"github.com/gooseek/backend/pkg/metrics"
 )
 
 type LLMLimitsConfig struct {
-	UsageRepo *usage.Repository
+	UsageRepo   *usage.Repository
+	EmailSender *email.Sender
 }
 
 func LLMLimits(config LLMLimitsConfig) fiber.Handler {
@@ -42,6 +45,8 @@ func LLMLimits(config LLMLimitsConfig) fiber.Handler {
 				}
 				metrics.RecordRateLimitHit("llm-svc", clientIP, reason)
 
+				sendLimitEmail(config, userID, tier, limits.LLMRequestsPerDay, limits.LLMRequestsPerDay)
+
 				return c.Status(429).JSON(fiber.Map{
 					"error":        reason,
 					"tier":         tier,
@@ -50,12 +55,68 @@ func LLMLimits(config LLMLimitsConfig) fiber.Handler {
 					"upgradeUrl":   "/settings/billing",
 				})
 			}
+
+			checkLimitWarning(config, userID, tier)
 		}
 
 		return c.Next()
 	}
 }
 
+func checkLimitWarning(config LLMLimitsConfig, userID, tier string) {
+	if config.UsageRepo == nil || config.EmailSender == nil || !config.EmailSender.IsConfigured() {
+		return
+	}
+
+	go func() {
+		todayUsage, err := config.UsageRepo.GetTodayUsage(context.Background(), userID)
+		if err != nil || todayUsage == nil {
+			return
+		}
+
+		limits := usage.GetLimits(tier)
+		if limits.LLMRequestsPerDay == 0 {
+			return
+		}
+
+		percentage := (todayUsage.LLMRequests * 100) / limits.LLMRequestsPerDay
+		if percentage >= 80 && percentage < 100 {
+			userEmail := getUserEmail(config.UsageRepo, userID)
+			if userEmail != "" {
+				if err := config.EmailSender.SendLimitWarning(userEmail, "", todayUsage.LLMRequests, limits.LLMRequestsPerDay, tier); err != nil {
+					log.Printf("[email] Limit warning send error: %v", err)
+				}
+			}
+		}
+	}()
+}
+
+func sendLimitEmail(config LLMLimitsConfig, userID, tier string, usageCount, limitCount int) {
+	if config.EmailSender == nil || !config.EmailSender.IsConfigured() {
+		return
+	}
+
+	go func() {
+		userEmail := getUserEmail(config.UsageRepo, userID)
+		if userEmail != "" {
+			if err := config.EmailSender.SendLimitWarning(userEmail, "", usageCount, limitCount, tier); err != nil {
+				log.Printf("[email] Limit exceeded email error: %v", err)
+			}
+		}
+	}()
+}
+
+func getUserEmail(repo *usage.Repository, userID string) string {
+	if repo == nil {
+		return ""
+	}
+	emailAddr, err := repo.GetUserEmail(context.Background(), userID)
+	if err != nil {
+		return ""
+	}
+	return emailAddr
+}
+
 type UsageTracker struct {
 	repo *usage.Repository
 }