feat: add combined environment endpoint for concurrent weather + air quality fetch

- POST /volatile/fetch/environment/{city} fetches both in parallel
- Single geocode lookup shared between API calls
- Uses asyncio.gather() for concurrent external requests
- Fix scheduler executor name (rest_api → rest_api_executor)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/clients/scheduler_client.py

@@ -18,7 +18,7 @@ class SchedulerTask(BaseModel):
 
     task_name: str = Field(..., description="Unique task identifier")
     service: str = Field(default="library-desk", description="Service that owns this task")
-    executor: str = Field(default="rest_api", description="Executor type")
+    executor: str = Field(default="rest_api_executor", description="Executor type")
     priority: int = Field(default=50, ge=1, le=100, description="Priority (lower = higher)")
     description: Optional[str] = Field(None, description="Human-readable description")
     enabled: bool = Field(default=True, description="Whether task is enabled")
@@ -291,7 +291,7 @@ class SchedulerClient:
         task = SchedulerTask(
             task_name=task_name,
             service="library-desk",
-            executor="rest_api",
+            executor="rest_api_executor",
             priority=60,  # Background maintenance priority
             description=description or f"Prefetch {namespace}/{key} for {user}",
             minute=schedule.get("minute", -1),