fix(x402-metrics): align Prometheus retention with recording-rule windows; rename mis-named lifetime rule

[bussyjd]

Why

27e1ac5 (this PR's parent) added recording rules over [24h] and [7d]. The Prometheus retention was 6h. increase(x[24h]) over a 6h TSDB silently returns 6h extrapolated as 24h — the frontend reads it and shows the wrong number with no warning.

Same commit's x402:revenue:lifetime_by_offer rule is sum(counter), not sum(increase[lifetime]) — it resets on every replica restart, so "lifetime" is a misnomer.

Before

   recording rule expressions      |   actual TSDB retention
   ---------------------------------------------------------
   increase(x[24h])                |   <- reads last 6h only
   increase(x[7d])                 |   <- reads last 6h only
   sum(charged_requests_total)     |   <- survives, but resets
                                   |     on every verifier restart

   Frontend "24h revenue" / "7d earnings" / "lifetime" cells all wrong

After

   recording rule expressions      |   actual TSDB retention
   ---------------------------------------------------------
   increase(x[24h])                |   <- 8d window, valid
   increase(x[7d])                 |   <- 8d window, valid
   sum(charged_requests_total)     |   <- renamed to
   -> total_by_offer_current       |     `total_by_offer_current`
                                   |     with doc-comment that says
                                   |     "this resets on rollout"

   Cells match reality. Naming matches semantics.

What changed

• monitoring.yaml.gotmpl: retention: 6h -> 8d (= 7d max window + 1d safety)
• x402-prometheus-rules.yaml: rename rule + add explanatory comment

Disk impact

~32x increase in Prometheus disk footprint (proportional to retention). Local-only kube-prometheus-stack PVC currently has no size cap (inherits storageClass default). Watch for PVC-full on next obol stack up and add a cap if it bites.

Frontend consumer

obol-stack-front-end/src/lib/services/prometheus.ts (lines 49, 61) references x402:revenue:lifetime_by_offer in a doc comment + OR-fallback expression for the lifetime-earnings query. Architecture-review notes flagged the OR fallback added in #330 was unused on the consumer side anyway, so this rename has zero immediate UI impact. A follow-up small PR on the frontend repo should update the name + comment to x402:revenue:total_by_offer_current. Not modified in this PR (separate repo).

Test plan

• go build ./... clean
• go test ./internal/embed/... ./internal/x402/... green
• On next stack up: PromQL x402:revenue:7d_by_offer_chain should return non-extrapolated values

Stacks on

PR #513. Rebase onto main after #513 merges.

[bussyjd]

Superseded by bundle PR #536 — closing in favor of the consolidated merge target. Original branch and history preserved.