feat(status): REPORTING_EPOCH — launch day becomes a one-env-var event #5553

Implements the owner's reporting policy ("accurate honest reporting starts once we are customer ready — the truth always wins") as a single switch:

• REPORTING_EPOCH (ISO date) is the date /status measures from. Uptime windows clamp to it — "90d uptime" over 18 recorded days is 100%-of-18-days, never a claim about the other 72 (effectiveWindowMs, floored at 1h so a just-set epoch can't zero the denominator). Incidents fully before the epoch leave both the math and the history table (open incidents always count). Days-since caps at the epoch's age. The measurement explainer prints the actual date.
• Unset, it falls back to the incident ledger's birth (2026-08-09) — nothing changes today, and no figure ever claims time nobody was recording. Garbage values fall back rather than silently measuring from 1970.

At customer-launch: set REPORTING_EPOCH=<launch date> in /opt/gluecron/.env, deploy, and the page starts the clean era — build-time history excluded, honestly labeled.

Three new tests pin the clamp, the floor, and the fallback chain. Typecheck clean; status suites 35 pass; design gate green.