feat: implement multi-identifier architecture (Issue #496)

- Database: Add user_login_ids table for 1:N identifier mapping and remove legacy login_id column
- Kratos: Update identity schema to use custom_login_ids array instead of a single id trait
- Backend: Implement syncCustomLoginIDs to collect isLoginId fields across tenant schemas
- Backend: Add backtracking logic to auto-assign session tenant based on used login identifier
- Backend: Add 409 Conflict exception handling for Create/Update operations
- AdminFront: Refactor UserDetailPage to a tabbed grid layout (Info, Tenants, Security)
- AdminFront: Show '로그인 ID' badge on tenant schema fields used for authentication
- UserFront: Remove legacy optional 'Login ID' input from signup flow
- Tests: Add multi-identifier repository tests and update handler tests

docker/ory/kratos/identity.schema.json

@@ -7,13 +7,16 @@
     "traits": {
       "type": "object",
       "properties": {
-        "id": {
-          "type": "string",
-          "title": "ID",
-          "ory.sh/kratos": {
-            "credentials": {
-              "password": {
-                "identifier": true
+        "custom_login_ids": {
+          "type": "array",
+          "title": "Custom Login IDs",
+          "items": {
+            "type": "string",
+            "ory.sh/kratos": {
+              "credentials": {
+                "password": {
+                  "identifier": true
+                }
               }
             }
           }