feat(security): stop the app origin serving anything it does not recognise, and make the uploads origin configurable (#103)

The uploads directory is the only place in this application where content someone else authored is served over HTTP. #95 stopped a dangerous file being stored; this stops a stored file doing damage if one ever gets there anyway — through a gap, a path added later, a restore, or a file written before that validation existed.

Two halves, complementary rather than alternative.

The app's own origin now serves uploads defensively. An allowlist of the three extensions the upload path can produce, so a `.html` or a `.svg` on disk is simply not a file this application hands out — 404, the same answer as a file that is not there, so the response cannot be used to learn which paths exist. An allowlist rather than a denylist because a denylist has to anticipate every type a browser might execute, which is a moving target across browsers and years, while this only has to know three. The content type is stated explicitly from that same list rather than sniffed or guessed from a name someone else chose, paired with `nosniff`. `default-src 'none'; sandbox` gives a directly-navigated file no capabilities at all, which is the only way one of these can do harm — an `<img>` embed does not execute script. Writes get 405 rather than falling through to a 404 that suggests the path is wrong.

The other half is the separate origin, which is the real fix, because the origin is the whole unit of trust in a browser. That needs a hostname and a certificate, which live outside this repository, so what is here is the switch: `UPLOADS_BASE_URL`, sent to the frontend at runtime through `/api/config` and joined onto stored paths by `uploadUrl`. Empty means the app's own origin, which is the default and what local development has, so nothing changes until it is pointed somewhere.

Stored paths stay site-relative. A stored value outlives any hostname baked into it, and rewriting them would be a migration to undo the day the hostname changes.

Runtime rather than built in, so one image serves every environment — the same reason `paypalClientId` and `demoMode` are already there. `UPLOADS_BASE_URL` has a line in `docker-compose.prod.yml` while still empty, deliberately: a Portainer stack variable with no line there is substituted into the file and never reaches the container, which is exactly how `UPLOADS_DIR` went missing on 2026-08-23.

Unset warns at boot, in the same shape as the admin gate — a working configuration with one defence switched off is worth saying out loud. Set without a scheme is refused outright, because a bare hostname joins onto a stored path as if it were relative and breaks every image on the site rather than failing visibly.

The compose guard now resolves `${VAR:-default}` to its default, which is what the container actually receives when the stack variable behind it is unset. A bare `${VAR}` is still left opaque, so a required variable referenced that way goes on counting as present — that check is about the line existing, not about the stack being filled in.

Closes #103

backend/src/uploadTypes.ts

@@ -37,6 +37,22 @@ const EXTENSION_FOR_TYPE: Readonly<Record<string, string>> = {
   'image/webp': '.webp'
 };
 
+/**
+ * The content type a stored file should be served as, from its extension.
+ *
+ * The inverse of `extensionFor`, and derived from the same record so the two
+ * cannot drift. Returns null for anything else, which is what lets the uploads
+ * route refuse to serve a file it does not recognise — the case that matters is
+ * a file written before this validation existed, or one that arrived through a
+ * gap, since nothing the current upload path accepts can produce another
+ * extension.
+ */
+export function typeForExtension(extension: string): string | null {
+  const lowered = extension.toLowerCase();
+  const found = Object.entries(EXTENSION_FOR_TYPE).find(([, ext]) => ext === lowered);
+  return found?.[0] ?? null;
+}
+
 export function isAllowedImageType(mimetype: string): boolean {
   return ALLOWED_IMAGE_TYPES.includes(mimetype);
 }