feat(tunnels): switch to temporary Cloudflare tunnels

Replace persistent Cloudflare tunnels (API-based) with temporary tunnels using
'cloudflared tunnel --url'. This removes the need for Cloudflare API tokens,
DNS records, and persistent tunnel management.

Changes:
- Install cloudflared binary in API Dockerfile
- Add start_cloudflared_tunnel() and stop_cloudflared_tunnel() to docker.py
- Update instance start/stop/restart/delete to use temporary tunnels
- Store tunnel PID in tunnel_id field, temporary URL in url/public_url
- Remove Cloudflare API service (cloudflare_tunnel.py)
- Remove cloudflared container from docker-compose
- Remove Cloudflare env vars (CLOUDFLARE_API_TOKEN, ZONE_ID, etc.)
- Remove Cloudflare configuration from config.py
- Remove Cloudflare startup check from main.py
- Remove /health/cloudflare endpoint

apps/api/Dockerfile

@@ -38,6 +38,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     "$(. /etc/os-release && echo "$VERSION_CODENAME")" stable" > /etc/apt/sources.list.d/docker.list \
     && apt-get update \
     && apt-get install -y --no-install-recommends docker-ce-cli docker-compose-plugin \
+    && curl -L --output /usr/local/bin/cloudflared https://github.com/cloudflare/cloudflared/releases/latest/download/cloudflared-linux-amd64 \
+    && chmod +x /usr/local/bin/cloudflared \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy dependencies from builder