Fix email-thread HTML injection, attachment path traversal, and missing authz (#475)

Hardens issues found in a security review of the current tree (separate from
the cookbook SSH PR):

- Email thread rendering (static/js/emailLibrary.js): the flat read path runs
  inbound HTML through the allowlist sanitizer, but the two threaded paths
  (_renderTurnsAsBubbles / _renderTurnsFromServer — the default view) injected
  server-parsed `body_html` raw into the DOM. A crafted inbound email could
  inject arbitrary markup (phishing/form/credential-capture/tracking; full XSS
  if a deployment relaxes the script CSP). Now sanitized on all paths.

- Attachment extraction (routes/email_routes.py, routes/email_helpers.py): the
  on-disk extraction dir was `ATTACHMENTS_DIR / f"{folder}_{uid}"` with
  user-controlled folder/uid and no containment, so a folder like `../../tmp`
  could escape ATTACHMENTS_DIR. New attachment_extract_dir() flattens both to a
  single safe segment and asserts containment.

- Diagnostics routes (routes/diagnostics_routes.py): /api/db/stats,
  /api/rag/stats, /api/test/youtube, /api/test-research relied only on the
  global session check (any logged-in user). Now require_admin-gated.

- Defense-in-depth HTML escaping: session HTML export escapes the session name
  (routes/session_routes.py); the MCP OAuth page escapes the reflected Host
  header / server_id (routes/mcp_routes.py).

- Internal-tool token now compared with secrets.compare_digest (constant time)
  in core/middleware.py and app.py.

Adds regression tests in tests/test_security_regressions.py.

app.py

@@ -21,6 +21,7 @@ import uuid
 
 import asyncio
 import logging
+import secrets
 from datetime import datetime
 from typing import Dict
 
@@ -222,7 +223,7 @@ if AUTH_ENABLED:
             try:
                 from core.middleware import INTERNAL_TOOL_HEADER, INTERNAL_TOOL_TOKEN as _ITT
                 _hdr = request.headers.get(INTERNAL_TOOL_HEADER)
-                if _hdr and _hdr == _ITT and _is_trusted_loopback(request):
+                if _hdr and secrets.compare_digest(_hdr, _ITT) and _is_trusted_loopback(request):
                     # Impersonation: when the agent's loopback call sets
                     # X-Odysseus-Owner, attribute the request to that user only
                     # if they exist. Authorization checks remain separate; this