fix: use VERSION constant in _health endpoint

Add VERSION constant at top of pds.js and reference it in the health
check endpoint instead of hardcoded value.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

src/pds.js

···
 // ║ Environment bindings, SQL row types, protocol constants                      ║
 // ╚══════════════════════════════════════════════════════════════════════════════╝
 
+// PDS version (keep in sync with package.json)
+const VERSION = '0.3.0';
+
 // CBOR primitive markers (RFC 8949)
 const CBOR_FALSE = 0xf4;
 const CBOR_TRUE = 0xf5;
···
 
   // Health check endpoint
   if (url.pathname === '/xrpc/_health') {
-    return Response.json({ version: '0.1.0' });
+    return Response.json({ version: VERSION });
   }
 
   // Root path - ASCII art