feat(matching): implement 3-tier account system with fairness-based recording assignment

Add account tier system (BASIC/SUPPORTER/COMFORT) to reduce recording burden
for premium users while maintaining fairness through karma-based assignment.

Database Changes:
- Add AccountTier enum (BASIC, SUPPORTER, COMFORT)
- Add User.accountTier with BASIC default
- Add User.recordingsDone and User.recordingsReceived for karma tracking
- Add EventParticipant.accountTierOverride for event-specific tier upgrades
- Migration: 20251129220604_add_account_tiers_and_recording_stats

Matching Algorithm Updates:
- Implement fairness debt calculation: receivedCount - doneCount
- Apply tier penalties: SUPPORTER (-10), COMFORT (-50)
- New sorting priority: Location > Fairness > Load balancing
- Add getEffectiveTier() helper for tier resolution with override support
- Add getRecordingStatsForUsers() for fetching karma statistics

Tier Behavior:
- BASIC: Normal recording frequency (baseline, no penalty)
- SUPPORTER: Moderately reduced frequency (fairness penalty -10)
- COMFORT: Significantly reduced frequency (fairness penalty -50)
- All tiers can still be assigned when no better candidates available

Constants:
- ACCOUNT_TIER enum in src/constants/tiers.js
- FAIRNESS_SUPPORTER_PENALTY = 10
- FAIRNESS_COMFORT_PENALTY = 50

Tests:
- Update tests for dual buffer system semantics
- All 30 tests passing
- Fix imports: HEAT_BUFFER → HEAT_BUFFER_BEFORE

backend/src/constants/index.js

@@ -1,6 +1,10 @@
 const { MATCH_STATUS, SUGGESTION_STATUS } = require('./statuses');
+const { ACCOUNT_TIER, FAIRNESS_SUPPORTER_PENALTY, FAIRNESS_COMFORT_PENALTY } = require('./tiers');
 
 module.exports = {
   MATCH_STATUS,
   SUGGESTION_STATUS,
+  ACCOUNT_TIER,
+  FAIRNESS_SUPPORTER_PENALTY,
+  FAIRNESS_COMFORT_PENALTY,
 };