docs(website): the authoring UI proper, and the one route it needed (Phase 13)

EVENTS.md I gains what the phase settled -- the rule by which a form gives way to
the JSON box, why the condition builder is the engagement one rather than a
second one shaped like it, and the live cap meter as a fifth affordance beside
the dry run it must not be mistaken for. The screens table's two unfinished rows
are marked built; rehearsal is marked reachable, which it had not been since
Phase 10 shipped it.

The API surface table gains POST /admin/events/price, and a paragraph on why it
is a route rather than arithmetic in the browser: cost() is a server function of
params, and the dry run both dispatches and records, neither of which belongs
behind a keystroke.

EVENTS_PLAN.md carries Phase 13 as built, with the four decisions, the two
defects it closes in already-merged code, and the six things the build settled.

BACKEND_DESIGN.md's /events/* rows are Phase 1's surface and have not moved
since; a note says so and points at EVENTS.md as canonical rather than
re-listing eleven routes that file already owns.

Checks: docs has no CI. Every relative link and in-page anchor in the new
sections was resolved by hand; the files are CRLF in the working tree and each
diff is content-sized (8/0, 64/5, 67/0), so none carries the \r\r\n full-file
rewrite.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

website/BACKEND_DESIGN.md

@@ -1709,6 +1709,14 @@ file a route sits in — that is the property the route manifest freezes.
 | GET | `/events/series` | the arcs a definition may belong to. Read-only in this phase |
 | — | `/shard/*` · `/uo-link/*` | **Served by `module-uo`, not by core** (33 routes). Documented in [`../modules/uo/API.md`](../modules/uo/API.md) |
 
+> **The `/events/*` rows above are the surface as of Phase 1**, and they are not the whole of it.
+> The live run controls (Phase 3), the calendar and series writes (Phase 4), `advance` (Phase 5),
+> the dry run and the action switchboard (Phase 6), the option-source route (Phase 7), `cleanup`
+> (Phase 8) and the cap meter (Phase 13) are not listed here. **[`EVENTS.md`](EVENTS.md) § API
+> surface is the canonical table** and carries every one of them with the reasoning for its gate;
+> re-listing them here would be a second copy of a contract that file owns, and the copy that
+> drifts is always the second one.
+
 Every admin write logs to `activity_log`.
 
 ### The SPA HTML shell  (`app.js` → `utils/htmlShell.js`)