fix(server): Cache-Control no-cache on /static — heuristic caching served stale bundles

The static handler sent no Cache-Control, so browsers fell back to
heuristic caching (10% of age since Last-Modified) and kept serving a
stale reader.js for hours after a deploy — fixed web bugs reported as
"not fixed". no-cache forces revalidation every load; the filesystem
Last-Modified/If-Modified-Since pair keeps unchanged loads cheap.

Found while closing out the PDF adjust v2 session: the user's Chrome
was running a pre-endpoint bundle, which read as "the web has not
been fixed".

internal/router/router.go

@@ -250,6 +250,21 @@ func RegisterRoutes(cfg *Config) *handlers.Handler {
 	registerWebSocketRoutes(cfg)
 	registerFrontendRoutes(cfg)
 	registerDocumentationRoutes(cfg)
+
+	// Static bundles are served unhashed (reader.js, index.js), so the
+	// browser must revalidate on every load — with no Cache-Control it
+	// fell back to heuristic caching and served stale bundles for hours
+	// after a deploy (user-visible: fixed web bugs "not fixed"). no-cache
+	// still allows cheap If-Modified-Since 304 revalidation.
+	e.Pre(func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(c *echo.Context) error {
+			if strings.HasPrefix(c.Request().URL.Path, "/static/") {
+				c.Response().Header().Set(echo.HeaderCacheControl, "no-cache")
+			}
+			return next(c)
+		}
+	})
+
 	e.Static("/static", "web/static")
 
 	// Catch-all 404 handler - must be last