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/note_routes.py

@@ -10,6 +10,7 @@ from fastapi import APIRouter, HTTPException, Request
 from pydantic import BaseModel
 
 from core.database import SessionLocal, Note
+from core.middleware import INTERNAL_TOOL_USER
 from src.auth_helpers import require_user
 from src.constants import DATA_DIR
 from sqlalchemy.orm.attributes import flag_modified
@@ -582,7 +583,7 @@ def setup_note_routes(task_scheduler=None):
         return require_user(request) or None
 
     def _is_admin_or_single_user(request: Request, user: str | None) -> bool:
-        if user == "internal-tool":
+        if user == INTERNAL_TOOL_USER:
             return True
         if not user:
             # require_user() already admitted this request, which only happens

routes/research_routes.py

@@ -12,6 +12,7 @@ from typing import Optional
 from fastapi import APIRouter, HTTPException, Query, Request
 from fastapi.responses import HTMLResponse, StreamingResponse
 from pydantic import BaseModel, Field
+from core.middleware import INTERNAL_TOOL_USER
 from src.endpoint_resolver import resolve_endpoint
 from src.auth_helpers import _auth_disabled, get_current_user
 from core.auth import RESERVED_USERNAMES
@@ -386,7 +387,7 @@ def setup_research_routes(research_handler, session_manager=None) -> APIRouter:
         """Launch a research job from the dedicated panel."""
         from src.auth_helpers import require_privilege
         user = require_privilege(request, "can_use_research")
-        if user == "internal-tool":
+        if user == INTERNAL_TOOL_USER:
             tool_owner = (request.headers.get("X-Odysseus-Owner") or "").strip()
             if tool_owner and tool_owner not in RESERVED_USERNAMES:
                 auth_mgr = getattr(request.app.state, "auth_manager", None)

routes/shell_routes.py

@@ -15,6 +15,7 @@ from collections import namedtuple
 from pathlib import Path
 from typing import Dict, Any
 from core.platform_compat import IS_APPLE_SILICON, which_tool
+from core.middleware import INTERNAL_TOOL_USER
 from src.optional_deps import prepare_optional_dependency_import
 
 # POSIX-only: `pty`/`fcntl` transitively import `termios`, which does NOT exist
@@ -55,7 +56,7 @@ def _require_admin(request: Request):
     # In-process tool loopback. The AuthMiddleware already validated the
     # internal token + loopback client before setting this marker, so
     # honour it here as admin-equivalent.
-    if user == "internal-tool":
+    if user == INTERNAL_TOOL_USER:
         return
     if not user or user == "api":
         raise HTTPException(403, "Admin only")

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