fix(api): editing an in-progress non-preset weekly/monthly plan is refused as a ramp change #1184

Summary

Saving an in-progress plan whose stored ramp is a non-preset weekly or monthly (for example weekly 20%/7d, created through the API) fails with 409 "cannot change the ramp schedule", even when only the name changes.

Evidence (origin/main with #1183)

• The frontend can only send the two presets or custom. After fix(plans): Edit pre-selects Immediate for weekly/monthly ramps with non-preset percents #1156 / fix(plans): prefill non-preset weekly and monthly ramps as custom in Edit #1183, Edit pre-fills such a plan as Custom with the same percent and interval, so Save sends type: custom, 20%, 7 days.
• rampScheduleShapeUnchanged (internal/api/handler_plans.go:431) compares the type, so weekly and custom count as a change even though percent, interval and step count are identical.
• For a plan with CurrentStep > 0, preserveRampProgress (internal/api/handler_plans.go:408-423) then refuses the update with 409.
• At CurrentStep == 0 the save succeeds, but the stored type becomes custom and the start and next-execution dates are rebuilt.

Before #1183 the form sent immediate for these plans, which also returned 409, so this is not a regression; #1183 made the behaviour visible.

Proposed fix

Treat two ramps as the same shape when percent, interval and total steps match, whatever the type label is, or keep the stored type when an equivalent custom ramp is submitted. Add handler tests for:

• a rename of an in-progress weekly 20% plan succeeds and keeps its progress;
• a real change of percent or interval is still refused.

Found by review of #1183.