feat(fase4): add GlobalVariable API routes

Add schemas and router for global variables with CRUD operations
and admin-only access controls for create/update/delete.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/api-gateway/app/schemas/global_variable.py

@@ -0,0 +1,33 @@
+from pydantic import BaseModel
+from typing import Optional
+from uuid import UUID
+from datetime import datetime
+
+
+class GlobalVariableCreate(BaseModel):
+    key: str
+    value: Optional[str] = None
+    value_type: str = "string"
+    description: Optional[str] = None
+    is_secret: bool = False
+
+
+class GlobalVariableUpdate(BaseModel):
+    value: Optional[str] = None
+    value_type: Optional[str] = None
+    description: Optional[str] = None
+    is_secret: Optional[bool] = None
+
+
+class GlobalVariableResponse(BaseModel):
+    id: UUID
+    key: str
+    value: Optional[str]
+    value_type: str
+    description: Optional[str]
+    is_secret: bool
+    created_at: datetime
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True