Program builder — rep ranges, load, RPE and rest per exercise

The first piece of the program-builder redesign (grounded in a 180-source review of the best coaching tools):

• Double progression by default. Each exercise now takes Sets, a rep range (min–max), target Load (kg), RPE, and Rest — the prescription style ~90% of coaches actually use (hit the top of the range, add load).
• It round-trips. Re-opening a program loads every field back; the client app shows the full prescription ("3 × 8–12 @ 80kg · RPE 8 · rest 120s").
• Foundation for what's next. Under the hood the model now supports rep ranges, load types, RPE/RIR, superset/AMRAP/EMOM group timing, coach substitution lists, and exercise synonyms — the groundwork for the mobile builder, offline client logging, and substitution coming next.