Increase Radxa E20C rootfs for Docker builds

[tacomilkshake]

Summary

• set rootfs_size_mb: 2048 for the Radxa E20C recipe
• raise Sensorbox ASU container MAX_CUSTOM_ROOTFS_SIZE_MB to 4096 so the recipe validates
• document the recipe-level rootfs_size_mb field
• update the firmware-selector submodule to pass recipe rootfs size through to ASU
• add no-store headers for selector assets and recipes so users do not keep submitting stale JS after deploys

Repro

1. Choose Radxa E20C
2. Add Orb deployment token
3. Set root password
4. Set Install to onboard eMMC true
5. Add Tailscale auth key
6. Enable Install Docker
7. Build image

Before this change, the build fails during ImageBuilder root.ext4 creation with:

Build failed.
Error: Selected packages exceed device storage
imagebuilder_status=failed
error: ext4_allocate_best_fit_partial: failed to allocate 4654 blocks, out of space?
RuntimeError: Selected packages exceed device storage

The failed ASU request had rootfs_size_mb: null, so ImageBuilder used the default small rootfs even though the target E20C devices have 16 GB eMMC.

Validation

Validated on Ranch with Radxa E20C + eMMC install + Tailscale + Docker selected. The rebuilt request included rootfs_size_mb: 2048 and completed successfully.

Depends on firmware-selector PR: orbforge/firmware-selector-openwrt-org#2