feat(secrets): owner-discriminated AAD (SecretOwner) for group secrets

The crypto foundation for group-owned secrets (the §5.3 'single hardest
correctness detail'), done first and proven before the storage/resolution
layer.

- SecretOwner{App(AppId)|Group(GroupId)}; secret_aad/seal/open take the
  owner. The app AAD is byte-identical to the pre-Phase-3
  'secret:{app_id}:{name}', so every existing v1 row decrypts unchanged;
  group secrets use a disjoint 'secret:group:{group_id}:{name}' namespace
  (the 'group:' infix separates app/group AAD even for equal UUIDs).
- All callers (SDK get/set, secrets_api, apply email-secret read) wrapped
  in SecretOwner::App — behavior identical for app secrets.
- New audit test aad_distinguishes_app_and_group_owner proves the two
  namespaces don't collide (both directions, even reusing the UUID); the
  existing cross-app/cross-name swap audit tests stay green.

16 secrets_service tests pass; clippy clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/lib.rs

@@ -208,7 +208,7 @@ pub use secrets_repo::{
     SecretsRepoError, StoredSecret,
 };
 pub use secrets_service::{
-    open as open_secret, seal as seal_secret, SecretsConfig, SecretsServiceImpl,
+    open as open_secret, seal as seal_secret, SecretOwner, SecretsConfig, SecretsServiceImpl,
     DEFAULT_SECRET_MAX_VALUE_BYTES,
 };
 pub use topic_repo::{PostgresTopicRepo, Topic, TopicAuthMode, TopicRepo, TopicRepoError};