fix(oauth): resolve server by path segment, not subdomain

Claude Desktop got past discovery + DCR but /oauth/authorize rejected
the resource parameter with invalid_resource. Root cause:
resolveServerByResource() extracted the slug from the URL's first
hostname label (subdomain routing), but production runs path routing —
mcp.buildmymcpserver.com/<slug>. The function saw resource
"https://mcp.buildmymcpserver.com/text-generation", tried to look up
slug="mcp", missed, returned null → 400.

Path lookup is now tried first (matches the production topology and
the resource URL we publish via /.well-known/oauth-protected-resource),
port lookup second (local dev), subdomain lookup last with an explicit
"mcp" guard so the legacy path doesn't shadow the new one.

apps/api/src/routes/oauth.ts

@@ -40,16 +40,42 @@ function pkceVerify(verifier: string, challenge: string, method: string): boolea
 
 async function resolveServerByResource(resource: string) {
   const url = new URL(resource);
-  const port = url.port ? Number(url.port) : null;
-  if (port !== null) {
-    const [s] = await db.select().from(mcpServers).where(eq(mcpServers.hostPort, port)).limit(1);
+
+  // Path routing (the prod topology on mcp.buildmymcpserver.com): the slug
+  // is the first path segment. Has to be checked BEFORE the subdomain
+  // heuristic below, otherwise we extract "mcp" from "mcp.example.com/<slug>"
+  // and look up the wrong (or no) server. Claude Desktop's RFC 8707 resource
+  // parameter matches the `resource` field we publish in /.well-known/
+  // oauth-protected-resource, which is exactly the path-routed public URL.
+  const firstSegment = url.pathname.split('/').filter(Boolean)[0];
+  if (firstSegment) {
+    const [s] = await db
+      .select()
+      .from(mcpServers)
+      .where(eq(mcpServers.slug, firstSegment))
+      .limit(1);
     if (s) return s;
   }
+
+  // Port-based lookup — used in local dev where the runner is reached
+  // directly at http://<RUNNER_HOST>:<port>.
+  const port = url.port ? Number(url.port) : null;
+  if (port !== null) {
+    const [s] = await db
+      .select()
+      .from(mcpServers)
+      .where(eq(mcpServers.hostPort, port))
+      .limit(1);
+    if (s) return s;
+  }
+
+  // Subdomain routing — legacy / future <slug>.mcp.example.com setup.
   const slug = url.hostname.split('.')[0];
-  if (slug) {
+  if (slug && slug !== 'mcp') {
     const [s] = await db.select().from(mcpServers).where(eq(mcpServers.slug, slug)).limit(1);
     if (s) return s;
   }
+
   return null;
 }