feat: revoke (unlink) device via DELETE /devices/:id

[Andreschuks101]

Revoke (unlink) a device — DELETE /devices/:id

closes #157

Soft-revokes a device and tears down all of its access across the cluster.

Changes

• Schema/migration: new devices (revokedAt, publicKey, lastSeenAt) and
  device_prekeys tables, relations, types, and migration 0007_device_revocation.
• DELETE /devices/:id: stamps revokedAt, deletes the device's prekeys,
  disconnects its live sockets, publishes device_revoked on the Redis bus, and
  emits a key_change notice to peers in shared conversations.
• GET /devices: lists the caller's devices.
• Revocation service: persistence runs in a transaction; the "last active
  device" rule is enforced atomically inside the UPDATE so concurrent revokes
  cannot remove a user's final device.
• Socket layer: sockets bind to a per-device room on connect; a revoked or
  foreign device is refused at connection time, excluding it from future fan-out.
  A Redis bus subscriber tears down device sockets on every instance.

Behaviour / acceptance criteria

• revokedAt set; prekeys removed; live sockets disconnected.
• Revoked device excluded from future fan-out (leaves its room, rejected on reconnect).
• Revoking the only active device returns 409.
• Peers in shared conversations receive a key_change event.

Testing

• New unit tests for the route and the revocation service (ownership, already-revoked,
  last-device 409, success path, concurrent-revoke race, socket disconnect, peer
  notification, bus publish).
• Full backend suite passes (109 tests); lint and prettier clean.

Notes

• Clients pass deviceId in the Socket.IO handshake auth to bind a socket to a device.
• Device/prekey registration endpoints are out of scope for this issue (revocation only).

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