feat(auth): enhance tenant resolution in JWT handling

Updated the authentication logic to prioritize tenant resolution from JWT claims, specifically using `tenants.bgp` or `bgp_tenant_id` as the primary source. If these claims are absent, the system will fallback to the configured `EVOBGP_PORTAL_TENANT_ID`. This change improves the flexibility of tenant management in the authentication process. Additionally, updated related documentation to reflect these changes and added tests to ensure proper functionality.

internal/httpapi/server.go

@@ -61,7 +61,7 @@ type Options struct {
 	JWTSecret      string // AUTH_JWT_SECRET / EVOBGP_AUTH_JWT_SECRET (HS256 shared secret)
 	AuthIssuer     string // AUTH_ISSUER (expected iss claim; default https://auth.shnt.top)
 	AuthPortalURL  string // AUTH_PORTAL_URL (returned by /v1/auth/config for the UI)
-	PortalTenantID string // EVOBGP_PORTAL_TENANT_ID (single tenant scope for JWT users)
+	PortalTenantID string // fallback when JWT has no bgp_tenant_id / tenants.bgp
 	AuthRequired   bool   // AUTH_REQUIRED / EVOBGP_AUTH_REQUIRED (surfaced via /v1/auth/config)
 }