Flip M7 Part 2 to landed (Android-app#15) and record the two implementation
decisions: the direct-ntfy embedded distributor (no UnifiedPush library — the
plan's stated likely path; foreground-service SSE, no second app, no Google
Play Services), and the small additive push.ntfyUrl settings field the app
needs to discover the relay (website#79). Document push.ntfyUrl +
NTFY_PUBLIC_URL in BACKEND_DESIGN.md.
Co-Authored-By: Claude <noreply@anthropic.com>