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/config/config.go

@@ -91,6 +91,16 @@ type Config struct {
 	ScrapeTimeout   time.Duration
 	SearchTimeout   time.Duration
 
+	// SMTP / Email
+	SMTPHost     string
+	SMTPPort     int
+	SMTPUser     string
+	SMTPPassword string
+	SMTPFrom     string
+	SMTPTLS      bool
+	SiteURL      string
+	SiteName     string
+
 	// CORS
 	AllowedOrigins []string
 }
@@ -174,6 +184,15 @@ func Load() (*Config, error) {
 		ScrapeTimeout: time.Duration(getEnvInt("SCRAPE_TIMEOUT_MS", 25000)) * time.Millisecond,
 		SearchTimeout: time.Duration(getEnvInt("SEARCH_TIMEOUT_MS", 10000)) * time.Millisecond,
 
+		SMTPHost:     getEnv("SMTP_HOST", ""),
+		SMTPPort:     getEnvInt("SMTP_PORT", 587),
+		SMTPUser:     getEnv("SMTP_USER", ""),
+		SMTPPassword: getEnv("SMTP_PASSWORD", ""),
+		SMTPFrom:     getEnv("SMTP_FROM", "GooSeek <noreply@gooseek.ru>"),
+		SMTPTLS:      getEnv("SMTP_TLS", "true") == "true",
+		SiteURL:      getEnv("SITE_URL", "https://gooseek.ru"),
+		SiteName:     getEnv("SITE_NAME", "GooSeek"),
+
 		AllowedOrigins: parseOrigins(getEnv("ALLOWED_ORIGINS", "*")),
 	}