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/database/models.py

@@ -253,6 +253,10 @@ class CloudSettings(Base):
     device_name = Column(String, nullable=True)
     account_user_id = Column(String, nullable=True)
     connected_at = Column(DateTime, nullable=True)
+    # Server-assigned sync device id (device table on the cloud side). Set when
+    # this install registers as an encryption-capable device; the matching
+    # X25519 private key lives in the OS keychain (services/cloud_keys.py).
+    sync_device_id = Column(String, nullable=True)
     updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)