v1.46.0 — Startup config guards (D1)

Two startup config guards (D1), closing the last code items from the Opus 5 pre-launch review.

Billing-mount guard — when FEEDECHO_BILLING_ENABLED=1, the app now verifies at startup that /api/billing/checkout is actually registered. If the billing module failed to mount (or the wrong entrypoint was served), the app refuses to boot instead of silently 302-ing every signup into a 404 and stranding them at a paused trial.

Credential-key hard-fail — a missing FEEDECHO_CREDENTIAL_KEY now raises in real multi mode (no sqlite fallback) instead of warning and booting with plaintext credentials. This closes the silent S3 (credential-encryption) undo path. Local development (sqlite fallback) keeps the warn-not-raise behavior, mirroring the DATABASE_URL carve-out.

Both guards were reviewed by Kimi K3 (1 test-only finding, fixed) and are live on feedecho.net as v1.46.0.