feat: fetch and consume device prekey bundle

[Andreschuks101]

Fetch + consume prekey bundle — GET /users/:userId/devices/:deviceId/key-bundle

closes #160

Serves a recipient device's public prekey bundle and consumes one one-time prekey
per fetch, so a sender can start an X3DH/Signal-style encrypted session.

Changes

• Schema/migration: devices (identity public key, registrationId, signed
  prekey + signature, revokedAt) and one_time_pre_keys (consumed flag,
  unique (device_id, key_id), (device_id, consumed) index). Migration
  0007_device_key_bundles. Only public key material is stored.
• Service fetchAndConsumeKeyBundle: looks up the device, returns 404 if it is
  unknown or revoked, and otherwise returns
  { identityPublicKey, registrationId, signedPreKey, oneTimePreKey | null }.
• Route: GET /users/:userId/devices/:deviceId/key-bundle, authenticated.

Atomic, race-free consumption

The one-time prekey is claimed in a single statement:

UPDATE one_time_pre_keys SET consumed = true
WHERE id = (SELECT id FROM one_time_pre_keys
            WHERE device_id = $1 AND consumed = false
            ORDER BY key_id FOR UPDATE SKIP LOCKED LIMIT 1)
RETURNING key_id, public_key

FOR UPDATE SKIP LOCKED ensures concurrent senders skip a row another transaction
is already claiming, so the same one-time prekey can never be issued twice. When
the pool is exhausted the bundle is still returned with oneTimePreKey: null.

Acceptance criteria

• One-time prekey consumption is atomic and race-free under concurrent fetches.
• Returns oneTimePreKey: null gracefully when the pool is exhausted.
• Revoked (and unknown) devices return 404.
• No private key material is ever stored or returned.

Testing

• New service and route tests: atomic/concurrent distinct-OTP behaviour, graceful
  null on exhaustion, 404 for unknown and revoked devices, and a "no private
  material" shape assertion.
• Full backend suite passes (108 tests); lint and prettier clean.

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