Match confirmed registrations to referral codes

Method signature: `Future matchRegistrationToCode(String newMemberId, String deviceFingerprint, String? sessionToken, DateTime registeredAt)`. Lookup strategy: first query by device_fingerprint, then by session_token if no fingerprint match found — union the results and take the most recent. Define the time window as a class-level constant `static const Duration _attributionWindow = Duration(hours: 72)` with a constructor parameter override for testing.

To prevent duplicates, add a UNIQUE constraint on attribution_records(new_member_id) at the DB level, and catch the unique violation in the Dart layer to return AlreadyMatched. The Supabase query should be: `.from('click_events').select().eq('device_fingerprint', fp).gte('clicked_at', cutoff).order('clicked_at', ascending: false).limit(1)`. After finding the click event, resolve mentor_id by joining referral_codes. Create the AttributionRecord in a single insert.

Use sealed classes for MatchResult: MatchFound(attributionId), NoMatchFound, AlreadyMatched.

Unit tests (flutter_test + Mockito): mock IRecruitmentAttributionRepository. Test cases must include: (1) single click event within window — attribution created; (2) two click events within window — most recent used; (3) click event at exactly 72h boundary — included (use inclusive comparison); (4) click event at 72h+1s — excluded; (5) no click events — NoMatchFound returned; (6) existing pending attribution — AlreadyMatched returned without DB insert; (7) repository insert failure — exception propagated, no partial state. Use fake DateTime injection to control 'now' in tests. Aim for 95% branch coverage.

Add a concurrency test using Future.wait with two simultaneous calls to confirm only one attribution record is created.