fix: send Cache-Control so a stale Nix mtime can't pin visitors to old JS

bridge.py

@@ -96,8 +96,17 @@ async def relay(request: web.Request) -> web.WebSocketResponse:
 # The client loads no inline script, no inline style and no foreign origin, so a
 # strict policy costs nothing. form-action stays 'self' rather than 'none' so the
 # compose dialog's method="dialog" form is untouched.
+#
+# Nix normalizes every built file's mtime to the same fixed epoch for
+# reproducibility, so Last-Modified is identical across every deploy — with no
+# explicit Cache-Control, a browser's heuristic freshness calculation (based on
+# that ~56-year-old timestamp) can treat a page as fresh indefinitely and never
+# revalidate again, silently pinning a visitor to whatever JS they first
+# loaded across every future deploy. no-cache forces a conditional GET each
+# time rather than disabling caching outright — the ETag aiohttp already sends
+# still turns an unchanged file into a bodyless 304.
 @web.middleware
-async def csp(request: web.Request, handler):
+async def headers(request: web.Request, handler):
     response = await handler(request)
     response.headers.setdefault("Content-Security-Policy", "; ".join([
         "default-src 'self'",
@@ -107,6 +116,7 @@ async def csp(request: web.Request, handler):
         "object-src 'none'",
         "frame-ancestors 'none'",
     ]))
+    response.headers.setdefault("Cache-Control", "no-cache")
     return response
 
 
@@ -128,7 +138,7 @@ def main() -> int:
     args = parser.parse_args()
 
     logging.basicConfig(level=logging.INFO, format="%(asctime)s %(levelname)s %(message)s")
-    app = web.Application(middlewares=[csp])
+    app = web.Application(middlewares=[headers])
     app["web_dir"], app["relay_ports"] = args.dir, set(args.allow_ports)
     app["allow_origins"] = set(args.allow_origins)
     app.router.add_get("/", index)