Video LoRA: reuse adapters across clips with the same set

The video model stays cached across clips, but LoRAs were loaded from disk
and fully unloaded on every clip — wasted I/O and fusion latency, since
consecutive clips of a match request the identical fighter+env LoRAs.

Cache the active LoRA signature on the pipe (_coderai_active_loras) and only
swap when it changes: a request with the same set reuses the loaded adapters,
a different set (or none) triggers a clean unload + reload. Replaces the
apply-then-unload-every-clip path.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>