feat(admin): add ActivityLog schema and isAdmin flag

Database changes:
- Add ActivityLog model for comprehensive activity tracking
  - Track all user actions (auth, events, matches, admin)
  - Denormalized username for query performance
  - JSON metadata for flexibility
  - Multiple indexes for common filter patterns
- Add isAdmin flag to User model for admin access control
- Add activityLogs relation to User

Schema pushed to database with prisma db push
Created initial admin user: spotlight@radziel.com

This is Phase 1 of Activity Log System implementation.
Next phases: backend service, middleware, API endpoints, frontend UI.

backend/prisma/schema.prisma

@@ -61,6 +61,9 @@ model User {
   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
 
+  // Admin flag
+  isAdmin                 Boolean      @default(false) @map("is_admin")
+
   createdAt               DateTime     @default(now()) @map("created_at")
   updatedAt               DateTime     @updatedAt @map("updated_at")
 
@@ -73,6 +76,7 @@ model User {
   eventParticipants       EventParticipant[]
   heats                   EventUserHeat[]
   recordingAssignments    RecordingSuggestion[] @relation("RecorderAssignments")
+  activityLogs            ActivityLog[]
 
   @@map("users")
 }
@@ -322,3 +326,42 @@ model MatchingRun {
   @@index([eventId, startedAt])
   @@map("matching_runs")
 }
+
+// Activity logs for admin monitoring
+model ActivityLog {
+  id           Int      @id @default(autoincrement())
+
+  // Core identification
+  userId       Int?     @map("user_id")       // NULL for system actions
+  username     String?  @db.VarChar(50)       // Denormalized for performance
+  ipAddress    String?  @map("ip_address") @db.VarChar(45) // IPv4 or IPv6
+
+  // Action details
+  action       String   @db.VarChar(50)       // 'auth.register', 'match.create', etc.
+  category     String   @db.VarChar(20)       // 'auth', 'event', 'match', 'admin', 'chat'
+  resource     String?  @db.VarChar(100)      // Resource ID (e.g., 'event:123', 'match:abc')
+
+  // Contextual data
+  method       String?  @db.VarChar(10)       // HTTP method: GET, POST, PUT, DELETE
+  path         String?  @db.VarChar(255)      // API path
+  metadata     Json?                          // Flexible JSON for action-specific data
+
+  // Result tracking
+  success      Boolean  @default(true)        // Track failures too
+  errorMessage String?  @map("error_message") @db.Text
+
+  // Timestamp
+  createdAt    DateTime @default(now()) @map("created_at")
+
+  // Relations
+  user         User?    @relation(fields: [userId], references: [id], onDelete: SetNull)
+
+  // Indexes for performance
+  @@index([userId, createdAt])
+  @@index([action, createdAt])
+  @@index([category, createdAt])
+  @@index([createdAt(sort: Desc)])  // Most common query pattern
+  @@index([username, createdAt])    // Search by username
+
+  @@map("activity_logs")
+}