feat(analytics): enrich PostHog VIN/parts/subscription events (SASE-PH-001 Faz 0/1 additive subset)
Per sase-posthog-events-prd.md, additive-only pass (no renames, no dead-feature events):
- search.tsx: add search_input_focused, search_input_validation_failed,
search_paste_detected, search_history_item_selected; track query_source
(manual|paste|history) and propagate to vin_decoded/success/error; add
response_time_ms, source, status_code on success/error; add selected_index,
candidates_count, time_to_select_ms to vin_decode_candidate_selected.
- parts-panel.tsx: add parts_panel_viewed with vehicle_id, category_id,
parts_count, has_prices, available_groups_count (one-shot per mount).
- subscription/index.tsx: enrich plan_selected (period, from_plan, status),
checkout_started (from_plan, brands_count, trial_available, status),
subscription_cancelled (from_plan, billing_period, status_before_cancel,
days_since_start, had_downgrade_offer).
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