Add frontend and Caddy to Docker Compose dev environment

Containerize the frontend dev server (Vite) and add a Caddy reverse
proxy for HTTPS termination, replacing the manual local setup. The
Caddy config proxies /api/* to the backend and everything else to the
frontend dev server.

Also simplify start.sh: remove --local Poetry mode, extract
get_compose_cmd helper, and document new services and DEV_HOST env var.

crawler/frontend/Caddyfile.dev

@@ -0,0 +1,11 @@
+{$DEV_HOST:localhost}:443 {
+    tls internal
+
+    handle /api/* {
+        reverse_proxy app:5001
+    }
+
+    handle {
+        reverse_proxy frontend:5173
+    }
+}