Add custom peer ID support to register endpoint

- Update /register endpoint to accept optional custom peer ID
- Add validation: 1-128 chars, non-empty, must be unique
- Return 409 Conflict if peer ID already in use
- Remove outdated API.md documentation
- Update README.md with new register endpoint format

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

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

src/app.ts

@@ -64,11 +64,37 @@ export function createApp(storage: Storage, config: Config) {
   /**
    * POST /register
    * Register a new peer and receive credentials
+   * Accepts optional peerId in request body for custom peer IDs
    */
   app.post('/register', async (c) => {
     try {
-      // Generate new peer ID
-      const peerId = generatePeerId();
+      let peerId: string;
+
+      // Check if custom peer ID is provided
+      const body = await c.req.json().catch(() => ({}));
+      const customPeerId = body.peerId;
+
+      if (customPeerId !== undefined) {
+        // Validate custom peer ID
+        if (typeof customPeerId !== 'string' || customPeerId.length === 0) {
+          return c.json({ error: 'Peer ID must be a non-empty string' }, 400);
+        }
+
+        if (customPeerId.length > 128) {
+          return c.json({ error: 'Peer ID must be 128 characters or less' }, 400);
+        }
+
+        // Check if peer ID is already in use by checking for active offers
+        const existingOffers = await storage.getOffersByPeerId(customPeerId);
+        if (existingOffers.length > 0) {
+          return c.json({ error: 'Peer ID is already in use' }, 409);
+        }
+
+        peerId = customPeerId;
+      } else {
+        // Generate new peer ID
+        peerId = generatePeerId();
+      }
 
       // Encrypt peer ID with server secret (async operation)
       const secret = await encryptPeerId(peerId, config.authSecret);