Add Syndicate Invite Token System (Secure Links)

- Added route + handler: GenerateInviteLinkHandler to create signed tokens
- Added handler: JoinSyndicateWithTokenHandler to join using invite token
- Integrated secure token generation via helpers.GenerateSecureToken()
- Created DB model: syndicate_invite_tokens (assumed pre-existing)
- Updated syndicate view template to allow managers to generate links
- Flash messaging for invite success/failure
- Invite links are scoped to manager role and valid for 48 hours

helpers/token.go

@@ -0,0 +1,15 @@
+package helpers
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+)
+
+func GenerateSecureToken() (string, error) {
+	b := make([]byte, 32)
+	_, err := rand.Read(b)
+	if err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(b), nil
+}