chore: drop asuracomic.net from the userscript, CORS allowlist and docs (#97)

Closes #96.

## What

Removes every reference that still invites a Reader onto `asuracomic.net`.
The domain's deep links 301 to the `asurascans.com` **root**, discarding the
path (re-checked 2026-07-25), so a page on it never yields a series document
client-side and a stored address on it never yields a series page server-side.
#95 already pinned each Site to one hostname, so the backend rejects such an
address cleanly; this is the cleanup around that.

| File | Change |
|---|---|
| `userscript/manga-bookmark.user.js` | drops the `@match`, narrows the asura adapter to `/(^\|\.)asurascans\.com$/` |
| `userscript/test/logic.test.js` | new test pinning the narrowed host match |
| `.env.example`, `docker-compose.yml` | origin dropped from the `ALLOWED_ORIGINS` default |
| `DEPLOY.md` | same, and the sample list gains the two novel origins it was missing |
| `backend/api_test.go` | CORS fixtures and round-trip seed move to `asurascans.com` |
| `README.md`, `AGENTS.md` | notes say the host is dropped, not "stays matched" |

## Behaviour

- A Reader landing on `asuracomic.net` gets no userscript UI. Previously the
  script loaded and could do nothing useful — the redirect had already
  discarded the path.
- A request whose `Origin` is `https://asuracomic.net` is no longer reflected
  by a deployment using the shipped defaults.
- No backend logic changed: the CORS rule, the address gate and the poller are
  untouched. `AllowedOrigins` is data, not code.

## Security invariant preserved

CORS still reflects `Origin` only when it appears in `ALLOWED_ORIGINS`, with
`GET,PUT,DELETE,OPTIONS` and a `204` preflight — `TestCORSPreflight` and
`TestCORSDisallowedOrigin` still pin both halves, now against a live origin.
This change only removes a value from the allowlist, which is a narrowing.

## Verification

- `go test ./...` — full backend suite green (real Postgres per package).
- `node --test test/*.test.js` — 66/66 green, up one from the new match test.

## Deploy note (does not happen on merge)

The live allowlist comes from the VPS `.env`, not from these defaults, so the
origin must be dropped there in the same deploy. The one-off row repair for any
stored `asuracomic.net` address is in #96.

Reviewed-on: #97
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

userscript/test/logic.test.js

@@ -120,6 +120,14 @@ test("asura.detect returns other for non-series paths", () => {
   assert.equal(asura.detect(loc("https://asurascans.com/bookmarks")).type, "other");
 });
 
+test("asura.matches accepts only asurascans.com", () => {
+  assert.equal(asura.matches({ hostname: "asurascans.com" }), true);
+  assert.equal(asura.matches({ hostname: "www.asurascans.com" }), true);
+  // Dead domain: deep links 301 to the asurascans.com root, discarding the path.
+  assert.equal(asura.matches({ hostname: "asuracomic.net" }), false);
+  assert.equal(asura.matches({ hostname: "asurascans.com.evil.example" }), false);
+});
+
 test("asura.latestChapterFromAnchors takes the highest and skips the First Chapter shortcut", () => {
   const best = asura.latestChapterFromAnchors([
     { href: "/comics/solo-leveling-059befe1/chapter/1", text: "Chapter 1" },