fix: drop Clear-Site-Data reset breaking mini app styles

The once-per-version Clear-Site-Data: "cache" header on the index
navigation raced the page's own CSS/JS subresource loads in the
Telegram WebView, intermittently evicting or aborting the main
stylesheet so the mini app rendered half-styled on mobile.

It also could not fix stale HTML: it only fires when the document
actually reaches the backend, never when the WebView serves a cached
page. The no-store HTML plus immutable content-hashed asset filenames
already guarantee freshness without clearing the cache, so remove the
reset header, its helpers, constants, and tests.

tests/test_webapp_assets.py

@@ -523,52 +523,6 @@ class WebAppAssetTests(unittest.IsolatedAsyncioTestCase):
         self.assertEqual(exc.exception.location, "/uploaded-icon.png")
         self.assertEqual(exc.exception.headers["Cache-Control"], "no-cache")
 
-    def test_webapp_cache_reset_version_changes_with_asset_set(self):
-        with patch.object(webapp_assets, "_resolve_app_version", return_value="v1.2.3"):
-            first = webapp_assets._webapp_cache_reset_version(
-                "subscription_webapp.11111111.css",
-                "subscription_webapp.min.22222222.js",
-            )
-            second = webapp_assets._webapp_cache_reset_version(
-                "subscription_webapp.33333333.css",
-                "subscription_webapp.min.22222222.js",
-            )
-
-        self.assertRegex(first, r"^[0-9a-f]{16}$")
-        self.assertNotEqual(first, second)
-
-    def test_webapp_cache_reset_header_is_sent_once_per_version(self):
-        request = SimpleNamespace(cookies={})
-        response = web.Response()
-
-        webapp_assets._apply_webapp_cache_reset_header(request, response, "abc123")
-
-        self.assertEqual(response.headers["Clear-Site-Data"], '"cache"')
-        cookie = response.cookies[webapp_assets.WEBAPP_CACHE_RESET_COOKIE_NAME]
-        self.assertEqual(cookie.value, "abc123")
-        self.assertEqual(cookie["path"], "/")
-        self.assertEqual(
-            cookie["max-age"],
-            str(webapp_assets.WEBAPP_CACHE_RESET_COOKIE_MAX_AGE_SECONDS),
-        )
-        self.assertTrue(cookie["httponly"])
-        self.assertTrue(cookie["secure"])
-        self.assertEqual(cookie["samesite"], "None")
-
-        cached_request = SimpleNamespace(
-            cookies={webapp_assets.WEBAPP_CACHE_RESET_COOKIE_NAME: "abc123"}
-        )
-        cached_response = web.Response()
-
-        webapp_assets._apply_webapp_cache_reset_header(
-            cached_request,
-            cached_response,
-            "abc123",
-        )
-
-        self.assertNotIn("Clear-Site-Data", cached_response.headers)
-        self.assertNotIn(webapp_assets.WEBAPP_CACHE_RESET_COOKIE_NAME, cached_response.cookies)
-
     async def test_default_logo_route_serves_bundled_logo(self):
         settings = SimpleNamespace(WEBAPP_ENABLED=True)
         request = SimpleNamespace(app={"settings": settings})