Implement PromptHistoryRepository read/write interface

The idempotency of recordPromptSent is critical — the Rule Engine may be triggered multiple times for the same activity (e.g., app restarted, push notification retry). Implement this using a Supabase upsert with onConflict: 'activity_id,rule_id' and ignoreDuplicates: true, or a raw INSERT ... ON CONFLICT DO NOTHING. For hasPromptBeenSent, use a HEAD request with Prefer: count=exact to get just the count without fetching row data — this is the most efficient pattern for an existence check in Supabase PostgREST.

The mentorId parameter in fetchRecentPrompts should always be the currently authenticated user's UID (auth.currentUser.id) — do not accept arbitrary mentorIds from the UI layer, as this would bypass RLS intent. The windowHours parameter maps directly to the ScenarioRule.delayMaxHours: the Rule Engine queries prompt history within the maximum possible delay window to determine whether a prompt has already been sent during any valid delay slot.

Write unit tests in test/features/scenario_prompts/repositories/prompt_history_repository_test.dart using a mocked Supabase client. Cover: (1) recordPromptSent inserts and returns correct PromptHistoryRecord, (2) recordPromptSent called twice with same activityId+ruleId returns existing record (idempotency), (3) fetchRecentPrompts returns only records within the window (test with records inside and outside the window), (4) fetchRecentPrompts returns empty list when no recent prompts exist, (5) hasPromptBeenSent returns true when a matching record exists, (6) hasPromptBeenSent returns false when no matching record exists, (7) Supabase error maps to PromptHistoryRepositoryException. Write one integration test against local Supabase verifying the unique constraint and RLS policies.