fix(scrapers): login fast paths and re-auth hardening for httpx platforms (#357)

## Problem

On CSES, Kattis, and USACO, `:CP <platform> login` always prompted
for credentials and ran a full web login even when a valid session was
already cached. Submit also had weak stale-session detection.

## Solution

`credentials.lua` now tries cached credentials first before prompting,
delegating fast-path detection to each scraper. CSES `login()` checks
the cached API token and returns immediately if valid. USACO `login()`
and `submit()` call `_check_usaco_login()` upfront. Kattis `submit()`
emits `checking_login` consistently and also triggers re-auth on HTTP
400/403, not just on the `"Request validation failed"` text match.
The premature `Submitting...` log emitted by Lua before the scraper
started is removed — Python's own status events are sufficient.

scrapers/cses.py

@@ -248,7 +248,21 @@ class CSESScraper(BaseScraper):
         if not username or not password:
             return self._login_error("Missing username or password")
 
+        token = credentials.get("token")
         async with httpx.AsyncClient(follow_redirects=True) as client:
+            if token:
+                print(json.dumps({"status": "checking_login"}), flush=True)
+                if await self._check_token(client, token):
+                    return LoginResult(
+                        success=True,
+                        error="",
+                        credentials={
+                            "username": username,
+                            "password": password,
+                            "token": token,
+                        },
+                    )
+
             print(json.dumps({"status": "logging_in"}), flush=True)
             token = await self._web_login(client, username, password)
             if not token:
@@ -460,7 +474,8 @@ class CSESScraper(BaseScraper):
 
             if r.status_code not in range(200, 300):
                 try:
-                    err = r.json().get("message", r.text)
+                    body = r.json()
+                    err = body.get("error") or body.get("message") or r.text
                 except Exception:
                     err = r.text
                 return self._submit_error(f"Submit request failed: {err}")

scrapers/kattis.py

@@ -329,6 +329,7 @@ class KattisScraper(BaseScraper):
             return self._submit_error("Missing credentials. Use :CP kattis login")
 
         async with httpx.AsyncClient(follow_redirects=True) as client:
+            print(json.dumps({"status": "checking_login"}), flush=True)
             await _load_kattis_cookies(client)
             if not client.cookies:
                 print(json.dumps({"status": "logging_in"}), flush=True)
@@ -366,7 +367,7 @@ class KattisScraper(BaseScraper):
             except Exception as e:
                 return self._submit_error(f"Submit request failed: {e}")
 
-            if r.text == "Request validation failed":
+            if r.status_code in (400, 403) or r.text == "Request validation failed":
                 _COOKIE_PATH.unlink(missing_ok=True)
                 print(json.dumps({"status": "logging_in"}), flush=True)
                 ok = await _do_kattis_login(client, username, password)

scrapers/usaco.py

@@ -429,7 +429,19 @@ class USACOScraper(BaseScraper):
 
         async with httpx.AsyncClient(follow_redirects=True) as client:
             await _load_usaco_cookies(client)
-            if not client.cookies:
+            if client.cookies:
+                print(json.dumps({"status": "checking_login"}), flush=True)
+                if not await _check_usaco_login(client, username):
+                    client.cookies.clear()
+                    print(json.dumps({"status": "logging_in"}), flush=True)
+                    try:
+                        ok = await _do_usaco_login(client, username, password)
+                    except Exception as e:
+                        return self._submit_error(f"Login failed: {e}")
+                    if not ok:
+                        return self._submit_error("Login failed (bad credentials?)")
+                    await _save_usaco_cookies(client)
+            else:
                 print(json.dumps({"status": "logging_in"}), flush=True)
                 try:
                     ok = await _do_usaco_login(client, username, password)
@@ -470,7 +482,8 @@ class USACOScraper(BaseScraper):
                 headers=HEADERS,
                 timeout=HTTP_TIMEOUT,
             )
-            if "login" in page_r.url.path.lower() or "Login" in page_r.text[:2000]:
+            page_url = str(page_r.url)
+            if "/login" in page_url or "Login" in page_r.text[:2000]:
                 return self._submit_error("auth_failure")
             form_url, hidden_fields, lang_val = _parse_submit_form(
                 page_r.text, language_id
@@ -513,6 +526,16 @@ class USACOScraper(BaseScraper):
             return self._login_error("Missing username or password")
 
         async with httpx.AsyncClient(follow_redirects=True) as client:
+            await _load_usaco_cookies(client)
+            if client.cookies:
+                print(json.dumps({"status": "checking_login"}), flush=True)
+                if await _check_usaco_login(client, username):
+                    return LoginResult(
+                        success=True,
+                        error="",
+                        credentials={"username": username, "password": password},
+                    )
+
             print(json.dumps({"status": "logging_in"}), flush=True)
             try:
                 ok = await _do_usaco_login(client, username, password)