docs(arch): end-to-end system + tech-stack diagrams

New ARCHITECTURE.md pulls the desktop app (TECHNICAL.md) and the
license server (LICENSE-SERVER.md) into a single picture — the two
were never reconciled into an end-to-end view before.

Contents:
  §1. System diagram (ASCII) showing operator laptop, license
      server stack (nginx → FastAPI → Postgres), Postmark, Gumroad,
      and the buyer's machine — with the three primary flows
      (sale, manual mint, offline activation) traced through it.
  §2. Tech stack diagram, layered: desktop / server / operator /
      external SaaS, with version pins.
  §3. Trust + isolation boundaries table — what crosses each one
      and what the threat model is.
  §4. "Where things are stored" — paths, tables, files.
  §5. Pointers to the deeper per-component docs.

ASCII over Mermaid since the repo's Gitea version is unknown and
plain text renders in every viewer / IDE / raw `cat`.

LICENSE-SERVER.md status flipped from "design proposal, not built"
to "deployed (PR 1 + PR 2 code merged)" — that was stale since
the PR 1 deploy yesterday.

TECHNICAL.md and ADMIN.md gain one-line pointers to ARCHITECTURE.md
so people land at the unified view when looking for "how does it
all fit together".

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

docs/ADMIN.md

@@ -7,6 +7,8 @@ through the live server, where state lives on the box, how to rotate secrets,
 generating the signing keypair, the dev vs. production key story, and how to
 recover from key loss.
 
+For the end-to-end system + tech stack diagrams, see `ARCHITECTURE.md`.
+
 ---
 
 ## Live deployment (PR 1)