Update DSC-Brain + Check for Fleet Updates (GitHub release check)

DONE in PR #184. kit_update.py went from stub to real: github_latest() hits api.github.com/repos/weddas/DSC-HUB/releases/latest (Ethernet-gated, cached ~1h, offline-safe, 15-min fail back-off); fleet_firmware_status() diffs per-seat running ESP firmware vs EXPECTED_FIRMWARE (behind_count skips OOS). update_status() now returns brain{running,latest_tag,update_available} + fleet{diff}. check_updates() forces a fresh lookup. start_full_update() runs the operator-set brain_update_cmd as a background job, or returns the manual deploy instruction if unset (no blind git pull). New ‘Updates’ card on Settings -> Server: DSC-Brain row (chip + Check + Update behind DecisionLayer) + Fleet firmware table + ‘Reflash fleet’ reusing the existing ESPHome OTA rollout. 4 tests, full brain suite 261 passed. Follow-up: could merge with the ESPHome toolchain card (that’s the build-tool version; this is the DSC release/firmware version).