docs(product): success signals for the pilot in docs/product/metrics.md #113

Goal

Gap from the add-on audit #65 (customer-engagement checklist #16: "Erfolgsmessung ab Beta (docs/product/metrics.md): 3–5 messbare Signale aus dem Brief ableiten"). The brief has a "Success (measurable)" section and #107 publishes eval figures, but there is no metrics page that names the signals, their target values and how each is measured during the pilot (M2).

Acceptance criteria

• docs/product/metrics.md with 3–5 signals derived from docs/product/project-brief.md → Success, e.g. share of header fields correct without correction, handling time per request (protocol), export errors per week, model cost per request (docs(ops): AI cost limits per request and month, with a provider budget alert #109), p95 processing time
• Per signal: target value (from the proposal §7 or marked "to be set at kickoff"), source (eval, audit events, logs, protocol), how often it is read
• Every signal that cannot be measured yet is marked as such – no estimated figures
• Linked from the roadmap (M2) and the evidence page (docs(showcase): evidence page with measured, reproducible figures (#76) #107)

Security/Privacy affected?

No – aggregated counts only.