feat: extend the http registry to store account's installations

[kaichaosun]

Summary

Adds an account → device directory so an account can be resolved to its set of installation/device keys. Extends the existing HTTP keypackage registry to store and serve these account-to-device mappings.

Changes

• Account directory abstraction (account_directory.rs): introduces SignedDeviceBundle, DeviceSet, the AccountAuthority (account-key signing capability) and AccountDirectory (untrusted publish/fetch client) traits, plus the bundle codec and signature verification.
• Account traits (account.rs, service_traits.rs): LogosAccount implements AccountAuthority alongside IdentityProvider; on testnet the account key and device key coincide.
• Inbox/context integration (inbox_v2.rs, context.rs): publish_device_bundle upserts this installation's device key into the account's signed bundle (lamport-versioned, re-published idempotently); exposed via register_account_bundle.
• HTTP registry (bin/keypackage-registry/*, extensions/components/contact_registry*): new endpoints + store layer to persist and retrieve signed account installation bundles.

Related to #111

[jazzz]

Looks good for the current goals.

Notes for the Future:

• The account_id structure will need an update, however it will still contain the verifying key so minimal changes.
• Id recommend keeping the account and keypackage paths as separated as possible, as the code/functionality will be updated at different times