feat(scraper): add LoginResult model and abstract login() interface

Problem: `:CP <platform> login` blindly caches credentials without
validating them against the platform.

Solution: add `LoginResult` to `models.py`, abstract `login()` method
and `_login_error` helper to `BaseScraper`, and wire up the `"login"`
CLI dispatch in `_run_cli_async`.

scrapers/base.py

@@ -9,6 +9,7 @@ from .language_ids import get_language_id
 from .models import (
     CombinedTest,
     ContestListResult,
+    LoginResult,
     MetadataResult,
     SubmitResult,
     TestsResult,
@@ -58,9 +59,12 @@ class BaseScraper(ABC):
         credentials: dict[str, str],
     ) -> SubmitResult: ...
 
+    @abstractmethod
+    async def login(self, credentials: dict[str, str]) -> LoginResult: ...
+
     def _usage(self) -> str:
         name = self.platform_name
-        return f"Usage: {name}.py metadata <id> | tests <id> | contests"
+        return f"Usage: {name}.py metadata <id> | tests <id> | contests | login"
 
     def _metadata_error(self, msg: str) -> MetadataResult:
         return MetadataResult(success=False, error=msg, url="")
@@ -82,6 +86,9 @@ class BaseScraper(ABC):
     def _submit_error(self, msg: str) -> SubmitResult:
         return SubmitResult(success=False, error=msg)
 
+    def _login_error(self, msg: str) -> LoginResult:
+        return LoginResult(success=False, error=msg)
+
     async def _run_cli_async(self, args: list[str]) -> int:
         if len(args) < 2:
             print(self._metadata_error(self._usage()).model_dump_json())
@@ -133,6 +140,16 @@ class BaseScraper(ABC):
                 print(result.model_dump_json())
                 return 0 if result.success else 1
 
+            case "login":
+                creds_raw = os.environ.get("CP_CREDENTIALS", "{}")
+                try:
+                    credentials = json.loads(creds_raw)
+                except json.JSONDecodeError:
+                    credentials = {}
+                result = await self.login(credentials)
+                print(result.model_dump_json())
+                return 0 if result.success else 1
+
             case _:
                 print(
                     self._metadata_error(

scrapers/models.py

@@ -64,6 +64,12 @@ class TestsResult(ScrapingResult):
     model_config = ConfigDict(extra="forbid")
 
 
+class LoginResult(ScrapingResult):
+    credentials: dict[str, str] = Field(default_factory=dict)
+
+    model_config = ConfigDict(extra="forbid")
+
+
 class SubmitResult(ScrapingResult):
     submission_id: str = ""
     verdict: str = ""