feat: Sprint 10+11 — privacy hardening and multi-device support

Privacy Hardening (Sprint 10):
- Server --redact-logs flag: SHA-256 hashed identity prefixes in audit
  logs, payload_len omitted when enabled
- Client /privacy command suite: redact-keys on|off, auto-clear with
  duration parsing, padding on|off for traffic analysis resistance
- Forward secrecy: /verify-fs checks MLS epoch advancement,
  /rotate-all-keys rotates MLS leaf + hybrid KEM keypair
- Dummy message type (0x09): constant-rate traffic padding every 30s,
  silently discarded by recipients, serialize_dummy() + parse support
- delete_messages_before() for auto-clear in ConversationStore

Multi-Device Support (Sprint 11):
- Device registry: registerDevice @24, listDevices @25, revokeDevice @26
  RPCs with Device struct (deviceId, deviceName, registeredAt)
- Server storage: devices table (migration 008), max 5 per identity,
  E029_DEVICE_LIMIT and E030_DEVICE_NOT_FOUND error codes
- Device cleanup integrated into deleteAccount transaction
- Client REPL: /devices, /register-device <name>, /revoke-device <id>

72 core + 35 server tests pass.

schemas/node.capnp

@@ -123,6 +123,15 @@ interface NodeService {
   # A tombstone entry is added to the KT log for auditability.
   # All active sessions for the identity are invalidated.
   deleteAccount @23 (auth :Auth) -> (success :Bool);
+
+  # Register a device for the authenticated identity. Max 5 devices per identity.
+  registerDevice @24 (auth :Auth, deviceId :Data, deviceName :Text) -> (success :Bool);
+
+  # List all registered devices for the authenticated identity.
+  listDevices @25 (auth :Auth) -> (devices :List(Device));
+
+  # Revoke (remove) a registered device.
+  revokeDevice @26 (auth :Auth, deviceId :Data) -> (success :Bool);
 }
 
 struct Auth {
@@ -131,6 +140,12 @@ struct Auth {
   deviceId @2 :Data;    # optional UUID bytes for auditing/rate limiting
 }
 
+struct Device {
+  deviceId @0 :Data;
+  deviceName @1 :Text;
+  registeredAt @2 :UInt64;
+}
+
 # A delivery envelope pairing a per-inbox sequence number with an opaque payload.
 # Clients sort by `seq` before processing to guarantee MLS commit ordering.
 struct Envelope {