fix: extension updates install the new build — no 12h-cached "latest" XPI, and the popup's Update opens FC's install page

Operator, 2026-09-25: "the extension update trigger from inside the
extension doesn't work and the manual update seems to not move it to the
most recent version or at least mark it the most recent."

- fabledcurator-latest.xpi was served with Quart's default
  `public, max-age=43200`: one URL whose bytes change every release, so a
  browser that had fetched it reinstalled the previous build for 12 hours
  (measured on the instance). It is now `no-cache` (the ETag keeps an
  unchanged file a 304); versioned XPIs are `immutable`.
- The web Settings card installs/downloads the VERSIONED xpi_url, which can
  only ever be that build's bytes.
- The popup's Update button did tabs.create() on the .xpi, which Firefox
  refuses (NS_ERROR_FAILURE on a 200: it only installs from a user click on
  a web page). It now opens FC's install card (/subscriptions?tab=settings).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/frontend.py

@@ -46,6 +46,14 @@ async def serve_extension(filename: str):
 
     The application/x-xpinstall MIME tells Firefox to show its native
     install prompt instead of downloading the file as a blob.
+
+    Caching differs by name, and has to. A versioned name is one build's bytes
+    forever, so it can be cached for good. `fabledcurator-latest.xpi` is ONE
+    URL whose bytes change on every release, and Quart's default for a file
+    is `public, max-age=43200`: a browser that fetched it once reused those
+    bytes for 12 hours, so "install the latest" quietly reinstalled the
+    previous build (operator-flagged 2026-09-25). It is `no-cache` — the ETag
+    still makes an unchanged file a cheap 304.
     """
     if not _XPI_NAME_RE.fullmatch(filename):
         abort(404)
@@ -56,10 +64,11 @@ async def serve_extension(filename: str):
         if not xpis:
             abort(404)
         latest = xpis[-1]
-        return await send_file(
+        resp = await send_file(
             latest, mimetype="application/x-xpinstall",
             attachment_filename=latest.name,
         )
+        return _cache(resp, "no-cache")
     target = (XPI_DIR / filename).resolve()
     try:
         target.relative_to(XPI_DIR)
@@ -67,10 +76,19 @@ async def serve_extension(filename: str):
         abort(404)
     if not target.is_file():
         abort(404)
-    return await send_file(
+    resp = await send_file(
         target, mimetype="application/x-xpinstall",
         attachment_filename=filename,
     )
+    return _cache(resp, "public, max-age=31536000, immutable")
+
+
+def _cache(resp, policy: str):
+    """Set the XPI's Cache-Control, dropping the Expires send_file adds so the
+    two can never disagree."""
+    resp.headers["Cache-Control"] = policy
+    resp.headers.pop("Expires", None)
+    return resp
 
 
 @frontend_bp.route("/")