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/prisma/schema.prisma

@@ -11,6 +11,13 @@ datasource db {
   url      = env("DATABASE_URL")
 }
 
+// Account tier enum
+enum AccountTier {
+  BASIC
+  SUPPORTER
+  COMFORT
+}
+
 // Users table
 model User {
   id                      Int       @id @default(autoincrement())
@@ -47,9 +54,15 @@ model User {
   failedLoginAttempts     Int       @default(0) @map("failed_login_attempts")
   lockedUntil             DateTime? @map("locked_until")
 
-  avatar                  String?   @db.VarChar(255)
-  createdAt               DateTime  @default(now()) @map("created_at")
-  updatedAt               DateTime  @updatedAt @map("updated_at")
+  avatar                  String?      @db.VarChar(255)
+
+  // Account Tier & Recording Stats
+  accountTier             AccountTier  @default(BASIC) @map("account_tier")
+  recordingsDone          Int          @default(0) @map("recordings_done")    // How many times this user recorded others
+  recordingsReceived      Int          @default(0) @map("recordings_received") // How many times others recorded this user
+
+  createdAt               DateTime     @default(now()) @map("created_at")
+  updatedAt               DateTime     @updatedAt @map("updated_at")
 
   // Relations
   messages                Message[]
@@ -189,12 +202,13 @@ model Rating {
 
 // Event participants (tracks which users joined which events)
 model EventParticipant {
-  id               Int      @id @default(autoincrement())
-  userId           Int      @map("user_id")
-  eventId          Int      @map("event_id")
-  competitorNumber Int?     @map("competitor_number") // Bib number - one per user per event
-  recorderOptOut   Boolean  @default(false) @map("recorder_opt_out") // Opt-out from being a recorder
-  joinedAt         DateTime @default(now()) @map("joined_at")
+  id                   Int          @id @default(autoincrement())
+  userId               Int          @map("user_id")
+  eventId              Int          @map("event_id")
+  competitorNumber     Int?         @map("competitor_number") // Bib number - one per user per event
+  recorderOptOut       Boolean      @default(false) @map("recorder_opt_out") // Opt-out from being a recorder
+  accountTierOverride  AccountTier? @map("account_tier_override") // Override user's global tier for this event (e.g., Comfort Pass)
+  joinedAt             DateTime     @default(now()) @map("joined_at")
 
   // Relations
   user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)