feat(config): add language version selection with setup-time validation

Problem: users cannot choose which C++ standard (e.g. C++17 vs C++20
vs C++23) to submit with — the plugin hardcodes a single platform ID
per language with no discoverability or override mechanism.

Solution: add `LANGUAGE_VERSIONS` and `DEFAULT_VERSIONS` tables in
`constants.lua` as the single source of truth. `config.lua` gains
`version` and `submit_id` fields on `CpLanguage`/`CpPlatformOverrides`,
validated at setup time. `submit.lua` resolves the effective config to
a platform ID (priority: `submit_id` > `version` > default). Helpdocs
document the new `*cp-submit-language*` section with per-platform
available versions and the `submit_id` escape hatch. Tests cover
`LANGUAGE_IDS` completeness and `get_language_id` lookups.

tests/test_scrapers.py

@@ -1,5 +1,6 @@
 import pytest
 
+from scrapers.language_ids import LANGUAGE_IDS, get_language_id
 from scrapers.models import (
     ContestListResult,
     MetadataResult,
@@ -137,3 +138,28 @@ def test_scraper_metadata_error(run_scraper_offline, scraper, contest_id):
     assert objs
     assert objs[-1].get("success") is False
     assert objs[-1].get("error")
+
+
+EXPECTED_PLATFORMS = {"atcoder", "codeforces", "cses", "usaco", "kattis", "codechef"}
+EXPECTED_LANGUAGES = {"cpp", "python"}
+
+
+def test_language_ids_coverage():
+    assert set(LANGUAGE_IDS.keys()) == EXPECTED_PLATFORMS
+    for platform, langs in LANGUAGE_IDS.items():
+        assert set(langs.keys()) == EXPECTED_LANGUAGES, f"{platform} missing languages"
+        for lang, lid in langs.items():
+            assert isinstance(lid, str) and lid, f"{platform}/{lang} empty ID"
+
+
+@pytest.mark.parametrize("platform", EXPECTED_PLATFORMS)
+@pytest.mark.parametrize("language", EXPECTED_LANGUAGES)
+def test_get_language_id(platform, language):
+    result = get_language_id(platform, language)
+    assert result is not None, f"No ID for {platform}/{language}"
+    assert result == LANGUAGE_IDS[platform][language]
+
+
+def test_get_language_id_unknown():
+    assert get_language_id("nonexistent", "cpp") is None
+    assert get_language_id("codeforces", "rust") is None