v1.2.0:4 — make avg. watts a first-class SetLog field

Average watts (assault bike, rower, ski erg) was a free-text entry stuffed
into the per-set customMetrics JSON blob. Promote it to a real nullable
column, SetLog.watts, written through every set path (create / PATCH /
add-sets / import-save / account-import) and shown everywhere as
"Avg. watts" with a proper numeric input.

The column is added by the boot-time guarded ALTER in docker_entrypoint.sh
(additive, idempotent), so the version migration stays empty. Existing data
is untouched: legacy watts values remain readable from customMetrics and
migrate to the column the next time a set is saved.

proof-of-work/lib/exerciseOptions.ts

@@ -46,6 +46,7 @@ export const BASE_TRACKING_FIELDS: Option[] = [
   { value: "duration", label: "Time" },
   { value: "distance", label: "Distance" },
   { value: "calories", label: "Calories" },
+  { value: "watts", label: "Avg. watts" },
   { value: "notes", label: "Notes" },
 ];