fix(audit-2026-06-11/H-D1): bind AES-GCM AAD on secrets + realtime signing key

At-rest secrets were AES-256-GCM sealed with no Associated
Authentication Data, so anyone with Postgres write access could
ciphertext-swap rows across apps (or rename via row edit) and the
decrypt would silently succeed under the wrong identity, returning
attacker-chosen plaintext. This breaks the cross-app isolation boundary
the moment DB write access is achieved.

Adds an AAD-bound envelope (v1) alongside the legacy no-AAD layout (v0),
discriminated by a per-row `version` column:

* shared::crypto — new encrypt_with_aad / decrypt_with_aad using
  aes_gcm::aead::Payload { msg, aad }. Originals retained for v0 reads.
  Tests: AAD round-trip, AAD-mismatch fails, empty-AAD round-trip.

* migration 0042 — adds `secrets.version SMALLINT NOT NULL DEFAULT 0`
  and `app_secrets.realtime_signing_key_version SMALLINT NOT NULL
  DEFAULT 0`. Existing rows stay v0; new writes are v1.

* secrets (SDK + admin API) — seal() now binds AAD =
  "secret:{app_id}:{name}" and emits v1; open() dispatches on version.
  StoredSecret gains a `version` field; SecretsRepo::set takes it.
  Both secrets_service::set and secrets_api::set_secret go through the
  v1 path. Tests prove a cross-app swap and a cross-name swap both
  surface Corrupted, and that a hand-built v0 row still decrypts.

* app_secrets (realtime signing key) — get_or_create_signing_key writes
  v1 with AAD = "app_secret:{app_id}:realtime_signing_key"; decode
  dispatches on version. Tests cover v0 decode, v1 round-trip, and v1
  decode-under-wrong-app failing.

* email-trigger inbound secret — kept on v0 (seal_legacy/open_legacy)
  and explicitly deferred: email_trigger_details has no version column
  and the trigger_id isn't known at seal time. The audit classes the
  email-trigger AAD gap as Medium; folded into v1.2's key-versioning
  pass per SECURITY_AUDIT.md.

* expected_schema.txt re-blessed by hand (no local Postgres) for the two
  new columns + migration 0042.

Also folds in a let-else clippy fix in auth_api.rs (login Argon2
semaphore acquire, from the H-B1 commit) and two cargo-fmt reflows.

No re-encryption sweep — v0 rows decrypt as-is; the sweep is deferred to
v1.2's key-versioning pass (audit "Notes on remediation methodology").

Audit ref: security_audit/03_crypto_secrets.md (H-D1).

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

crates/manager-core/tests/expected_schema.txt

@@ -64,6 +64,7 @@ table: app_secrets
   updated_at: timestamp with time zone NOT NULL default=now()
   realtime_signing_key_encrypted: bytea NULL
   realtime_signing_key_nonce: bytea NULL
+  realtime_signing_key_version: smallint NOT NULL default=0
 
 table: app_slug_history
   slug: text NOT NULL
@@ -303,6 +304,7 @@ table: secrets
   nonce: bytea NOT NULL
   created_at: timestamp with time zone NOT NULL default=now()
   updated_at: timestamp with time zone NOT NULL default=now()
+  version: smallint NOT NULL default=0
 
 table: topics
   app_id: uuid NOT NULL
@@ -708,3 +710,4 @@ constraints on triggers:
   0039: app user invitations unique pending
   0040: execution logs keep history
   0041: dead letters composite idx
+  0042: secrets envelope version