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/lib/logger.ts

@@ -0,0 +1,19 @@
+function ts(): string {
+  return new Date().toISOString()
+}
+
+export const logger = {
+  info(tag: string, message: string) {
+    console.log(`[botfights:${tag}] ${ts()} ${message}`)
+  },
+  warn(tag: string, message: string) {
+    console.warn(`[botfights:${tag}] ${ts()} ${message}`)
+  },
+  error(tag: string, message: string, err?: unknown) {
+    if (err) {
+      console.error(`[botfights:${tag}] ${ts()} ${message}`, err)
+    } else {
+      console.error(`[botfights:${tag}] ${ts()} ${message}`)
+    }
+  },
+}