refactor: structured logging across server

Create server/src/lib/logger.ts with info/warn/error methods that add
[botfights:tag] timestamps. Replace bare console.log/warn/error calls
in index.ts, seed.ts, routes/fights.ts, engine/queue.ts, engine/mock.ts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/seed.ts

@@ -1,4 +1,5 @@
 import '../src/db/index.js'
+import { logger } from './lib/logger.js'
 import { seedMockBots, seedMockFights } from './engine/mock.js'
 
 async function main() {
@@ -8,8 +9,8 @@ async function main() {
   await seedMockBots()
   await seedMockFights(15)
 
-  console.log('[botfights] seed complete!')
+  logger.info('seed', 'seed complete!')
   process.exit(0)
 }
 
-main().catch(console.error)
+main().catch((err) => logger.error('seed', 'seed failed', err))