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.

routes/email_helpers.py

@@ -269,6 +269,20 @@ COMPOSE_UPLOADS_DIR.mkdir(parents=True, exist_ok=True)
 SCHEDULED_DB = DATA_DIR / "scheduled_emails.db"
 
 
+def attachment_extract_dir(folder: str, uid: str) -> Path:
+    """Containment-safe extraction directory for an attachment.
+
+    `folder` and `uid` are user-controlled (query/path params). Flatten them to
+    a single safe path segment so a value like folder='../../tmp' can't escape
+    ATTACHMENTS_DIR, then assert containment as belt-and-suspenders."""
+    key = re.sub(r"[^A-Za-z0-9._-]", "_", f"{folder}_{uid}") or "_"
+    target = (ATTACHMENTS_DIR / key).resolve()
+    base = ATTACHMENTS_DIR.resolve()
+    if target != base and base not in target.parents:
+        raise HTTPException(400, "Invalid attachment location")
+    return target
+
+
 def _init_scheduled_db():
     import sqlite3
     conn = sqlite3.connect(SCHEDULED_DB)