fix(azure): flag historical purchase amounts recorded in lookback-window units #669

Split out of #612 (item 4 of its checklist). Cap bypass and pending-execution refusal were closed by #655.

Problem

Azure purchases executed before the corrected provider pin (#475) and migration 000106 recorded lookback-window amounts, not term totals:

• purchase_history.upfront_cost (from result.Cost, internal/purchase/execution.go:945-960)
• purchase_executions.total_upfront_cost
• savings in history and dashboard totals (internal/api/handler_history.go ~1121, internal/api/handler_dashboard.go ~713 YTD)

Monthly-variant purchases also stored a non-zero upfront. The term price was never stored, so these rows cannot be backfilled numerically.

Proposed design (additive flag, no backfill)

1. Migration (next free number, 108 or later; 107 is taken by feat(ladder): planned-purchase timeline with filters and graph editing #665): nullable amount_basis (or boolean legacy_azure_window_amounts) on purchase_history, set for provider = 'azure' rows with timestamp before the cutover.
2. History API: expose the flag on history rows (handler_history.go ~1121).
3. Dashboard: exclude flagged rows from YTD savings totals, or include them with an annotation (handler_dashboard.go ~713).
4. Frontend: a "pre-fix estimate" badge on flagged rows.

Backend (1-3) and frontend (4) as separate PRs to stay under the 400-line limit.

Open owner questions (blocking)

1. Cutover timestamp: when migration 106 was applied in prod. Rows before it are flagged. Also needs rollout evidence that 106 was applied in dev and prod and a recollection ran.
2. Exclude vs annotate: should flagged rows be left out of YTD savings totals, or counted and annotated?

Not building until both are answered.