docs(modules): close Phase 4 — de-UO the backend reference, record the acceptance run

Phase 4 slice 4. BACKEND_DESIGN.md had never been de-UO'd: Phase 3 rewrote core's
code and README, but §5.2's identifier check reads code, not prose, so nothing
ever looked at the design document. It still described 27 shard_* tables, 20 UO
route rows and the shard visibility ladder as core's, a phase after core stopped
being able to serve any of them.

Moved, text unchanged:

  BACKEND_DESIGN §3   six shard_* schema sections (226 lines) → modules/uo/SCHEMA.md
  BACKEND_DESIGN §4   13 public + 7 admin UO route rows       → modules/uo/API.md
  BACKEND_DESIGN §6.5 the audience ladder (70 lines)          → modules/uo/API.md §4

Core keeps the seam and gains the eight /admin/modules routes it had never
documented. §6.5 becomes "Module-owned audience boundaries": core's security
boundary ends at authentication, roles and the session, and a module that serves
game data brings its own.

Also fixed on the way: users.router.js was still listed as 15 routes (it is 9 —
six went to the extension slot), and the push section still promised
config/shardStreams.js "moves out with it" four slices after it left.

The acceptance table now carries results. Criterion 2 was proved for real against
module-uo v0.3.0 on an empty database, which turned up the uninstall ordering
defect fixed in website#146.

AI disclosure: this contribution was AI-assisted (Claude Code).

Co-Authored-By: Claude <noreply@anthropic.com>

website/SPAWN_ATLAS.md

@@ -191,7 +191,7 @@ All are **import-owned**: a refresh empties and reloads them in one transaction,
 so a failed reload leaves the previous atlas intact rather than a half-loaded
 world. Nothing else writes to them and nothing holds a foreign key to them — no
 FKs at all, consistent with every other `shard_*` table. Full column listings in
-[`BACKEND_DESIGN.md`](BACKEND_DESIGN.md).
+[`../modules/uo/SCHEMA.md`](../modules/uo/SCHEMA.md) — these are module-uo's tables, not core's.
 
 | Table | Rows (stock) | Notes |
 |---|---|---|