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/cmd/llm-svc/main.go

@@ -15,6 +15,7 @@ import (
 	"github.com/gooseek/backend/internal/llm"
 	"github.com/gooseek/backend/internal/usage"
 	"github.com/gooseek/backend/pkg/config"
+	"github.com/gooseek/backend/pkg/email"
 	"github.com/gooseek/backend/pkg/metrics"
 	"github.com/gooseek/backend/pkg/middleware"
 	"github.com/gooseek/backend/pkg/ndjson"
@@ -189,12 +190,24 @@ func main() {
 		})
 	})
 
+	emailSender := email.NewSender(email.SMTPConfig{
+		Host:     cfg.SMTPHost,
+		Port:     cfg.SMTPPort,
+		User:     cfg.SMTPUser,
+		Password: cfg.SMTPPassword,
+		From:     cfg.SMTPFrom,
+		TLS:      cfg.SMTPTLS,
+		SiteURL:  cfg.SiteURL,
+		SiteName: cfg.SiteName,
+	})
+
 	llmAPI := app.Group("/api/v1", middleware.JWT(middleware.JWTConfig{
 		Secret:     cfg.JWTSecret,
 		AuthSvcURL: cfg.AuthSvcURL,
 		AllowGuest: false,
 	}), middleware.LLMLimits(middleware.LLMLimitsConfig{
-		UsageRepo: usageRepo,
+		UsageRepo:   usageRepo,
+		EmailSender: emailSender,
 	}))
 
 	llmAPI.Post("/generate", func(c *fiber.Ctx) error {