UwUSSH Server 0.1.1

Security fixes. A second look at 0.1.0 found ways to wear the server down, fill its disk, or
— from a directory someone else could write to — turn update.sh against its own machine.
Nothing that let anybody read a record or reach another account. Every one is fixed, most with a
test.

• Connections have deadlines and a ceiling. A client that opened a connection and then said
  nothing, or sent its headers a byte at a time, or sat idle after its last request, held a socket
  for as long as it liked; a few thousand of those were a server nobody else could reach. Now the
  headers of a request get fifteen seconds, an idle connection fifteen more, HTTP/2 connections are
  pinged and dropped when nobody answers, and the server takes 512 connections at once and 32 from
  one address (UWUSSH_MAX_CONNECTIONS, UWUSSH_MAX_CONNECTIONS_PER_IP; behind a proxy only the
  total counts). An event stream being answered is left alone. On Linux the server also raises its
  own open-file limit as far as the system allows.
• The disk cannot be filled through the backups. The quota was per account, and fourteen
  nightly backups each copied all of it. Now all accounts together hold at most
  UWUSSH_SERVER_MAX_MB (2 GiB by default), seven backups are kept instead of fourteen, and a
  backup that would leave the disk without room to spare is skipped with a warning. At the
  defaults, the database and its backups top out around 16 GiB.
• update.sh only trusts what only root can change. It looked for the server in the directory
  the shell was in, and ran whatever compose.yaml and .env it found there. Now it looks beside
  itself, in /opt/uwussh or where --dir says; refuses a directory, compose.yaml, .env or
  .uwussh-update that anybody but root (or the admin running sudo) could write to, or that sits
  under a directory someone else could; and refuses a .env that sets COMPOSE_… or DOCKER_…
  variables.
• A device holds at most eight session tokens. Signing in again and again piled up tokens in
  memory without end; now the oldest goes, and expired ones are swept every few minutes.
• A vault must be expensive to guess against. The server took a vault header asking for
  Argon2id at 1 KiB and one pass. It now wants at least 19 MiB and two passes for a new account or
  a new master password; the client uses 64 MiB and three. Vaults already on the server are not
  touched.

Manifests, for UwUSSH 0.1.1. The client now has each device publish a sealed list of what it
holds, so a device can tell when a server serves it old versions or holds records back. The server
stores them like any other record; it hands them only to clients that ask (manifests=1), so an
older client keeps syncing, and it drops a device's manifest when the device is revoked. UwUSSH
0.1.1 and this server belong together: update both.