feat(security): R4 S3 — abuse & leak hardening

- C-3.7: fold a per-link password_version into the signed watch grant (migration
  0059 adds the column) and bump it on every password change, so rotating a share
  link's password invalidates outstanding grants at once instead of letting them
  live out the 6h TTL. make_grant/check_grant take the version; update_link bumps.
- C-3.8: the share recipient picker and share-by-email now reuse the messageable
  filter (not-demo AND active AND not-suspended) instead of only excluding demo,
  so suspended/deactivated addresses aren't leaked or reachable as targets.
- C-3.9: GET /keys/{user_id} adopts get_thread's MB2 guard — a non-messageable
  target with no shared history returns the same "User not found" as a missing id,
  so it can't be probed to enumerate users or fetch suspended users' keys.
- C-3.12: _register_account catches IntegrityError on the insert (the select-then-
  insert TOCTOU) and treats it as the already-registered no-op.
- C-3.13: trigger_job claims the running flag atomically under _activity_lock
  (compare-and-set) instead of a bare check, closing the double-start window.
- Move _messageable/is_messageable_user into a shared app/userscope.py so downloads
  doesn't import the messages route module.

backend/app/auth.py

@@ -9,6 +9,7 @@ from fastapi import APIRouter, BackgroundTasks, Depends, HTTPException, Request
 from fastapi.responses import JSONResponse, RedirectResponse
 from starlette.requests import HTTPConnection
 from sqlalchemy import delete, func, select, update
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
 
 from app import email as email_mod
@@ -718,7 +719,14 @@ def _register_account(email: str, password: str) -> None:
             email_verified=not email_ok,
         )
         db.add(user)
-        db.flush()
+        try:
+            db.flush()
+        except IntegrityError:
+            # A concurrent registration for the same new email won the race between the pre-check
+            # above and this flush (the unique-email constraint fires). Same already-registered
+            # no-op as the pre-check hit — roll back and stop.
+            db.rollback()
+            return
         upsert_pending_invite(db, email)  # admin-approval gate (commits)
         if email_ok:
             raw = _issue_token(db, user, "verify", VERIFY_TTL)