refactor: Modularize server into separate concerns

Split monolithic index.ts (~1400 lines) into modular structure:
- config.ts: Server configuration and constants
- db/: Database initialization, migrations, and seeds
- routes/: API handlers by domain (themes, canvas, components, etc.)
- services/: Terminal WebSocket server
- utils/: CORS helpers

Entry point now only coordinates initialization.

server/routes/history.ts

@@ -0,0 +1,24 @@
+import { db } from '../db'
+import { jsonResponse } from '../utils/cors'
+
+export async function handleHistory(req: Request, url: URL) {
+  if (req.method === 'GET') {
+    const limit = parseInt(url.searchParams.get('limit') || '50')
+    const rows = db.query('SELECT * FROM history ORDER BY id DESC LIMIT ?').all(limit)
+    return jsonResponse(rows)
+  }
+
+  if (req.method === 'POST') {
+    const body = await req.json()
+    const stmt = db.prepare('INSERT INTO history (tool_name, args, result) VALUES (?, ?, ?)')
+    stmt.run(body.tool_name, JSON.stringify(body.args), body.result)
+    return jsonResponse({ success: true })
+  }
+
+  if (req.method === 'DELETE') {
+    db.run('DELETE FROM history')
+    return jsonResponse({ success: true })
+  }
+
+  return null
+}