P13-T05B — Android Keystore store + HTTP API adapter + Hilt

Status: LOCAL_DONE on codex/phase-13-free-accounts (phase PR #27, draft). Issue: #26 (slice 2 of P13-T05). Device evidence stays deferred to release per owner decision. No live backend contact (the preview base URL never resolves).

Slice acceptance (frozen before coding)

• T05B (this commit) — native Android boundary: AES/GCM session envelope (pure, JVM-tested), Keystore-backed AuthSessionStore with injectable key seam, OkHttp AuthAccountApi over the frozen backend paths with verbatim verdict mapping, Hilt AuthModule bindings. Zero new dependencies (framework KeyStore, pre-existing okhttp/org.json/mockwebserver/datastore) — nothing to license-review because nothing was introduced.
• T05C (next) — screens + app navigation wiring.
• Later — Keychain adapter + macOS run (release horizon).

What changed

• data/.../local/auth/SessionEnvelope.kt — envelope {"v":1,"iv","ct"} (12-byte IV, 128-bit tag, 64KiB cap); tamper/wrong-key/malformed/version/oversize all decode to null, never throw, so rehydrate lands logged-out.
• data/.../local/auth/KeystoreSessionStore.kt — AES key in AndroidKeyStore (API 26+, no user-auth gating that would strand background refresh), sealed blob in private prefs; SessionKeyProvider seam keeps JVM suites real (in-memory AES). The ~30 KeyStore boilerplate lines cannot run on JVM: statically reviewed + lint-guarded, behavior suites cover the rest.
• data/.../local/auth/AccountHttpApi.kt — blocking OkHttp over email/consume/refresh/revoke/link/deletion; account. prefix stripped to portable verdicts; transport/parse failures and non-envelope errors map to UNAVAILABLE (retryable, never an auth refusal); RFC3339 converted to epoch at the boundary.
• domain/.../portable/PortableAuth.kt — additive UNAVAILABLE const only.
• data/.../di/AuthModule.kt — Hilt singletons (prefs, keys, store, api with 10s timeouts, flow with wall clock + UUID nonces); base URL is an explicit .invalid preview placeholder.
• Tests: SessionEnvelopeTest (5: roundtrip, IV uniqueness, tamper, wrong key, malformed matrix), KeystoreSessionStoreTest (5: roundtrip, clear, rotation unreadable, corrupt, unavailable keystore), AccountHttpApiTest (7: request/consume/refresh/ revoke/delete/link shapes, 6-vector envelope matrix, transport+protocol failures).

Dependency review (recorded need/license/security)

• Added: none. okhttp 4.12.0 (Apache-2.0), org.json test stub and mockwebserver (same pins) pre-date this slice; AndroidKeyStore, java.time, javax.crypto are platform APIs (minSdk 26).
• Secret surface: scan-secrets.sh + PR-diff scan clean; no token or key material in logs, tests, or fixtures (synthetic values).

RED → GREEN

• RED proven by dropping the account. prefix strip: the 6-vector envelope matrix fails (full codes leak to UI mapping); GREEN on restore.

Validation (exact commands, this host)

./gradlew :domain:test :application:test :data:testDebugUnitTest :app:testDebugUnitTest :app:assembleDebug --no-daemon
make quick-verify
git diff --check

Outcome: 17 new data suites green (plus 16 T05A, baseline intact); full Android baseline + assembleDebug SUCCESS; quick-verify selection=full ok; diff --check clean; secrets scan clean.

Limits (not claimed)

• KeyStore boilerplate runs on device only (reviewed, not executed here); Keychain adapter + macOS run stay release.
• Preview base URL resolves nowhere; no screens yet (T05C).
• Live providers stay P13-T05/G13 device evidence (deferred).

Rollback

Pure addition under data/.../local/auth + data/.../di: deleting those files plus the UNAVAILABLE const restores the tree. No migration, no flag, no manifest change.

Next

P13-T05C screens + navigation (#26, same branch). Issue #26 stays open until the phase PR merges.