feat: enhance OIDC configuration for Azure login and improve error handling in API responses

Caddyfile

@@ -46,6 +46,13 @@
     }
 
     handle_errors {
+        # Don't mask API errors with the SPA shell — return a proper
+        # JSON error so the frontend can display a meaningful message.
+        @api path /api/*
+        respond @api `{"code":{err.status_code},"message":"Backend unavailable"}` {err.status_code} {
+            Content-Type application/json
+        }
+
         rewrite * /index.html
         file_server
     }

pb_migrations/1781000000_team_members_to_auth.js

@@ -47,6 +47,14 @@ migrate((app) => {
   }
 
   const tenant   = $os.getenv("ENTRA_TENANT_ID") || "common";
+  const clientId     = $os.getenv("ENTRA_CLIENT_ID");
+  const clientSecret = $os.getenv("ENTRA_CLIENT_SECRET");
+
+  const hasOidc = !!(clientId && clientSecret);
+  if (!hasOidc) {
+    console.log("WARN: ENTRA_CLIENT_ID / ENTRA_CLIENT_SECRET not set — OIDC provider will not be configured. " +
+                "Set the env vars and re-apply the migration to enable Azure login.");
+  }
 
   // 2. Recreate `team_members` as an auth collection (same name → all existing
   //    `user_id` references in test_results, on_demand_attempts,
@@ -75,7 +83,7 @@ migrate((app) => {
     mfa:          { enabled: false, duration: 600, rule: "" },
 
     oauth2: {
-      enabled: true,
+      enabled: hasOidc,
       // Map the OIDC userinfo claims onto our fields.
       mappedFields: {
         id: "",
@@ -83,18 +91,18 @@ migrate((app) => {
         username: "",
         avatarURL: "",
       },
-      providers: [
+      providers: hasOidc ? [
         {
           name: "oidc",
-          clientId:     $os.getenv("ENTRA_CLIENT_ID"),
+          clientId:     clientId,
-          clientSecret: $os.getenv("ENTRA_CLIENT_SECRET"),
+          clientSecret: clientSecret,
           authURL:     "https://login.microsoftonline.com/" + tenant + "/oauth2/v2.0/authorize",
           tokenURL:    "https://login.microsoftonline.com/" + tenant + "/oauth2/v2.0/token",
           userInfoURL: "https://graph.microsoft.com/oidc/userinfo",
           displayName: "Microsoft",
           pkce: true,
         },
-      ],
+      ] : [],
     },
 
     fields: [