Add cloud key store, API client, and sync identity flows

cloud_keys: device private key + master key live in the OS keychain
(keyring), namespaced per cloud account — never in the local DB. Headless
installs without a keychain get a hard error, not a plaintext fallback.

cloud_api: async httpx client for the bearer-key surface (devices,
account-key escrow, object push/commit, sync pull). Presigned blob
transfers use a separate unauthenticated client so the bearer key never
reaches the storage host.

cloud_account: the design-doc §9 flows — first-device setup mints MK +
recovery phrase (returned for one-time display), later devices register
and either adopt a wrapped MK provisioned by an existing device or
restore from the phrase. sync_device_id lands on cloud_settings with an
idempotent migration.

Tests run the real flows against an in-process fake cloud and assert the
master key and phrase never appear in any request body. A full round-trip
integration test (encrypt -> push -> pull -> decrypt -> tombstone) runs
against a live dev server when VOICEBOX_CLOUD_TEST_API/KEY are set.

backend/requirements.txt

@@ -7,9 +7,11 @@ pydantic>=2.5.0
 sqlalchemy>=2.0.0
 alembic>=1.13.0
 
-# Cloud backup/sync E2E encryption (services/cloud_crypto.py)
+# Cloud backup/sync E2E encryption (services/cloud_crypto.py); keyring stores
+# the device key + master key in the OS keychain (services/cloud_keys.py)
 pynacl>=1.5.0
 mnemonic>=0.21
+keyring>=25
 
 # ML models
 torch>=2.2.0