Make static files revalidate so the UI is fresh after updates

No Cache-Control header meant browsers cached JS/CSS heuristically for
days; after a server update (including the in-app self-update reload)
the backend was new but the frontend stayed stale. SEND_FILE_MAX_AGE
_DEFAULT=0 forces ETag revalidation — 304 when unchanged, fresh file
immediately after an update.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,10 @@ Version numbers follow [Semantic Versioning](https://semver.org/spec/v2.0.0.html
 
 ## [Unreleased]
 
+### Fixed
+
+- **Stale UI after updating the server** — Flask served `/static/` files with no `Cache-Control` header, so browsers cached JS/CSS heuristically (often for days). After a server update — including the new in-app self-update, whose post-install reload hit the cache — the backend was new but the frontend stayed old, and fixes appeared "not to work" until a hard refresh. `SEND_FILE_MAX_AGE_DEFAULT = 0` now makes every static file revalidate via ETag: unchanged files answer with a cheap 304, changed files are re-fetched immediately on the next normal page load.
+
 ---
 
 ## [1.7.4] — 2026-06-10