refactor(auth): centralize the internal-tool pseudo-username into a constant (#4333)

The in-process tool loopback stamps current_user = "internal-tool" and
require_admin grants admin to that sentinel; it is also a reserved username.
That security-sensitive string was hand-typed in ~7 places (stamp, admin gate,
RESERVED_USERNAMES, and standalone admin-equivalent checks in note/research/
shell/task routes), where a typo silently breaks an auth gate.

Add INTERNAL_TOOL_USER in core/middleware.py next to INTERNAL_TOOL_TOKEN/
INTERNAL_TOOL_HEADER and use it at every such site. A typo is now an
ImportError, not a silent mismatch. auth.py importing middleware is acyclic
(middleware imports no app modules). Behaviour is unchanged.

The multi-sentinel sets bundling internal-tool with api/demo/system
(assistant_routes, task_scheduler, research_routes) are a separate reserved-set
dedup, left for a follow-up.

Closes #4332

routes/task_routes.py

@@ -11,6 +11,7 @@ from fastapi import APIRouter, HTTPException, Request
 from pydantic import BaseModel
 
 from core.database import SessionLocal, ScheduledTask, TaskRun
+from core.middleware import INTERNAL_TOOL_USER
 from core.constants import internal_api_base
 from src.auth_helpers import get_current_user
 from src.constants import DATA_DIR, EMAIL_URGENCY_CACHE_DIR
@@ -427,7 +428,7 @@ def setup_task_routes(task_scheduler) -> APIRouter:
         # In-process tool-loopback marker — AuthMiddleware validated
         # the internal token + loopback client before stamping this,
         # so treat as admin-equivalent.
-        if user == "internal-tool":
+        if user == INTERNAL_TOOL_USER:
             return True
         try:
             from core.auth import AuthManager