P23-T01 — Favorites and justified opt-in return flows

Status: LOCAL_DONE on codex/phase-23-community-operations. Issue: #91. Entry G22 satisfied (P22 PR #90 merged cdd395f, issues #86–#89 CLOSED, wiki 12f75e8). Binds B-BR-C01–C06 and BUC-C01–C05. No migration, no backend change, no new permission, no iOS work.

What this task adds

• Duplicate-alert suppression for the existing local opt-in return flow (UC-014 vehicle price-drop alerts): the same drop notifies once per vehicle per evaluated week; repeat evaluations show nothing new; a new week re-arms. New domain port PriceDropAlertHistoryRepository (device-local only, no account/backend/PII), wired through EvaluatePriceDropAlertsUseCase, persisted in a dedicated Preferences DataStore (price_drop_alert_history) via a pure tested codec, bound in RepositoryModule/UseCaseModule. Clearing app data resets history (one repeat at most) — safe and explicit. Untouched: opt-in toggle, permission gating, cancel-on-disable, skip reasons, notification content.

Deliberately not built (no measured need)

• Follows, city/station activity subscriptions and backend push: not implemented — the local UC-014 alerts already cover the return journey and no measured need exists. Frozen contract for IF they ever are (must land before any client): per-account ownership with server-side derivation, one-tap unsubscribe, erasure covering subscriptions plus their alert history, keyset pagination; no endless feed or ranking incentives without evidence. Until that contract exists as code-backed tests, subscription code must not exist.
• Station favorites store: none exists to preserve or migrate; vehicle alert preferences are preserved byte-for-byte (no prefs schema change).

Validation

• EvaluatePriceDropAlertsUseCaseTest +2 (same-week silence, new-week notify + record), PriceDropAlertHistoryCodecTest 4 (round-trip, blank, isolation, stability).
• :domain:test 408 + :application:test 248 + :data:testDebugUnitTest 207 (1 pre-existing live-network skip), 0-fail (--rerun-tasks); :app:assembleDebug PASS (Hilt graph compiles).
• git diff --check clean; scoped secret review (local-only keys, random UUIDs, no PII/GPS).