fix(i18n): serve English for an unreadable catalog, refuse to boot without it

The server reads messages/<locale>.json from disk on first use. A missing
or corrupt file threw, and since the matchers iterate every locale, one bad
non-English catalog turned every request that touched them into a 500.
That locale is now served the English catalog, and the failure is logged
once as i18n.catalog.fallback with the locale and the reason. English is
the fallback itself: a process whose messages/en.json is missing or does
not parse refuses to start, with the file and the reason in the message.

The image build checks the catalogs twice with
scripts/verify-message-catalogs.mjs: in the builder against the locale
list in the source, and in the runner stage against the copies the image
carries.

scripts/verify-message-catalogs.mjs

@@ -0,0 +1,98 @@
+#!/usr/bin/env node
+/**
+ * Checks that every shipped locale has a message catalog that parses.
+ *
+ * The server reads `messages/<locale>.json` from disk at run time
+ * (`src/lib/i18n/server-catalogs.ts`); nothing is bundled, so an image that
+ * lost the directory, or one file of it, builds and starts clean and only
+ * shows the gap when a page asks for that locale. The Dockerfile runs this
+ * twice: in the builder against the locale list in `src/lib/i18n/config.ts`,
+ * writing that list out, and in the runner stage against the catalogs the
+ * image actually carries.
+ *
+ *   node scripts/verify-message-catalogs.mjs --dir messages \
+ *     --config src/lib/i18n/config.ts [--write .message-locales]
+ *   node verify-message-catalogs.mjs --dir /app/messages \
+ *     --locales .message-locales
+ *
+ * Exits 1 with every problem listed when a catalog is missing, does not parse
+ * or is not a JSON object, or when the locale list cannot be read.
+ */
+import { readFileSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+
+function arg(name) {
+  const index = process.argv.indexOf(`--${name}`);
+  return index === -1 ? null : (process.argv[index + 1] ?? null);
+}
+
+function localesFromConfig(path) {
+  const source = readFileSync(path, "utf8");
+  const block = /export const locales = \[([^\]]*)\]/.exec(source);
+  if (!block) throw new Error(`no \`export const locales = [...]\` in ${path}`);
+  const list = [...block[1].matchAll(/"([^"]+)"/g)].map((m) => m[1]);
+  if (list.length === 0) throw new Error(`the locale list in ${path} is empty`);
+  return list;
+}
+
+function localesFromFile(path) {
+  const list = readFileSync(path, "utf8").split("\n").filter(Boolean);
+  if (list.length === 0) throw new Error(`the locale list in ${path} is empty`);
+  return list;
+}
+
+const dir = arg("dir");
+const config = arg("config");
+const listFile = arg("locales");
+if (!dir || (!config && !listFile)) {
+  console.error(
+    "usage: verify-message-catalogs.mjs --dir <messages> (--config <config.ts> | --locales <file>) [--write <file>]",
+  );
+  process.exit(2);
+}
+
+let locales;
+try {
+  locales = config ? localesFromConfig(config) : localesFromFile(listFile);
+} catch (error) {
+  console.error(`ERROR: ${error.message}`);
+  process.exit(1);
+}
+
+const problems = [];
+for (const locale of locales) {
+  const path = join(dir, `${locale}.json`);
+  let text;
+  try {
+    text = readFileSync(path, "utf8");
+  } catch (error) {
+    problems.push(
+      `${path}: ${error.code === "ENOENT" ? "missing" : error.message}`,
+    );
+    continue;
+  }
+  try {
+    const parsed = JSON.parse(text);
+    if (
+      parsed === null ||
+      typeof parsed !== "object" ||
+      Array.isArray(parsed)
+    ) {
+      problems.push(`${path}: not a JSON object`);
+    }
+  } catch (error) {
+    problems.push(`${path}: does not parse (${error.message})`);
+  }
+}
+
+if (problems.length > 0) {
+  console.error(
+    "ERROR: message catalogs the server reads at run time are broken:",
+  );
+  for (const problem of problems) console.error(`  ${problem}`);
+  process.exit(1);
+}
+
+const write = arg("write");
+if (write) writeFileSync(write, `${locales.join("\n")}\n`);
+console.log(`message catalogs ok: ${locales.join(", ")} in ${dir}`);