docs: refresh architecture SVG FOUNDATION band for tier+payments+observability

The pipeline core of the diagram was still accurate, but the
FOUNDATION band predated the last few days of work:

- "quota tracking" -> "tier enforcement + payments" (Free/Pro/Business
  matrix + Lemon Squeezy scaffold, Days 42-43)
- added "Observability: Sentry + PostHog" — the Day 46 telemetry
  layer was entirely absent from the diagram

Surgical 2-line edit within the original character budget; no layout
reflow, pipeline visual untouched. SVG validated as well-formed XML.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: LEANDERANTONY