township: pre-fight intro clips, per-clip prompt regen, whole-match 2x enhance

- Every match now opens with 3 intro clips before the fight: a bold solo
  entrance for each fighter, then a referee-officiated face-off stare-down
  with the start signal. The real fight begins at clip 4. New intro prompt
  templates + LLM system prompt + PromptGenerator.intro_shot().
- Factor out _build_match_clip_specs() and _fill_clip_prompt() so all four
  clip-building paths (stage_videos Phase A, new-match, replan, full regen)
  build intros consistently; intro clips attach only their own participants
  (solo entrance = one fighter; face-off = both + match referee).
- New "clip-prompt" job scope + per-clip "prompt↻" link: rewrite ONLY one
  clip's prompt in place (role-aware), steering fight clips away from the
  match's other shots; renders nothing.
- "Create whole match" and "Regenerate whole match" (scope "full") now finish
  with a 2x AI upscale + 2x frame interpolation pass over the final short/long
  assemblies and outcome videos, reusing the existing enhance machinery.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>