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/models.py

@@ -943,6 +943,10 @@ class DownloadLink(Base, TimestampMixin):
         Boolean, default=False, server_default="false"
     )
     password_hash: Mapped[str | None] = mapped_column(Text)
+    # Bumped whenever the password changes; folded into the signed watch grant so rotating the
+    # password immediately invalidates any grants issued under the old one (instead of them living
+    # out the 6h grant TTL).
+    password_version: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
     expires_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     view_count: Mapped[int] = mapped_column(Integer, default=0, server_default="0")