feat(subscriptions,ui): delivery instrumentation — 24-hour counts and first-try success rate

[angela-helios]

Closes #586 — the second half of #580's operator page.

Stacked on #585 (base feat/580-subscriptions-ui): GitHub retargets to main when it merges, activating the Development link; the diff shrinks to just this feature then.

What the engine gains

A rolling ring of delivery counters per subscription (delivery_stats.rs): 48 half-hour, epoch-aligned buckets — deliberately the same shape as the dashboard's DashboardWindow::LastDay, so both surfaces bucket time identically. Outcomes are recorded at the single dispatch funnel every channel already runs through (dispatch_with_retry): success carries whether it landed on the first attempt; permanent errors, exhausted retries, and dispatch errors record as failures. Cost per delivery: one mutex-guarded bump.

In-memory by design. The window resets with the process, like the rest of the engine's volatile state, and the module docs say so. A durable delivery log (queryable across restarts) is separate, storage-backed work — deliberately out of scope.

What the page fills in

• The fourth card becomes DELIVERED IN 24 H, with the first-try success rate as its qualifier — absent when nothing was delivered, never a fabricated 100%.
• The table gains the design's LAST 24 HRS column between STATUS and SENT.
• Strings in en/es/de.

Verification

• Ring unit tests: accumulation, 24-hour age-out, slot reuse when a half-hour cycles around, removal.
• The UI test's injected provider drives the card sum (4,493) and the rate (96.2%) through the real render.
• Live end-to-end: five Encounters through two delivering channels put 10 on the card with the rate rendered — read back through the provider from the actual dispatch path.
• Full hermetic suite: 140 passed.