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.

crates/quicproquo-server/src/config.rs

@@ -33,6 +33,9 @@ pub struct FileConfig {
     pub federation: Option<FederationFileConfig>,
     /// Directory containing plugin `.so` / `.dylib` files to load at startup.
     pub plugin_dir: Option<PathBuf>,
+    /// When true, audit logs hash identity key prefixes and omit payload sizes.
+    #[serde(default)]
+    pub redact_logs: Option<bool>,
 }
 
 #[derive(Debug)]
@@ -55,6 +58,8 @@ pub struct EffectiveConfig {
     pub federation: Option<EffectiveFederationConfig>,
     /// Directory to scan for plugin `.so` / `.dylib` files at startup. None = no plugins.
     pub plugin_dir: Option<PathBuf>,
+    /// When true, audit logs hash identity key prefixes and omit payload sizes.
+    pub redact_logs: bool,
 }
 
 #[derive(Debug, Default, Deserialize)]
@@ -219,6 +224,7 @@ pub fn merge_config(args: &crate::Args, file: &FileConfig) -> EffectiveConfig {
     };
 
     let plugin_dir = args.plugin_dir.clone().or_else(|| file.plugin_dir.clone());
+    let redact_logs = args.redact_logs || file.redact_logs.unwrap_or(false);
 
     EffectiveConfig {
         listen,
@@ -235,6 +241,7 @@ pub fn merge_config(args: &crate::Args, file: &FileConfig) -> EffectiveConfig {
         metrics_enabled,
         federation,
         plugin_dir,
+        redact_logs,
     }
 }