feat(scraper): add precision extraction, start_time, and submit support

Problem: problem pages contain floating-point precision requirements and
contest start timestamps that were not being extracted or stored. The
submit workflow also needed a foundation in the scraper layer.

Solution: add extract_precision() to base.py and propagate through all
scrapers into cache. Add start_time to ContestSummary and extract it
from AtCoder and Codeforces. Add SubmitResult model, abstract submit()
method, submit CLI case with get_language_id() resolution, stdin/env_extra
support in run_scraper, and a full AtCoder submit implementation; stub
the remaining platforms.

scrapers/base.py

@@ -1,8 +1,31 @@
 import asyncio
+import json
+import os
+import re
 import sys
 from abc import ABC, abstractmethod
 
-from .models import CombinedTest, ContestListResult, MetadataResult, TestsResult
+from .language_ids import get_language_id
+from .models import CombinedTest, ContestListResult, MetadataResult, SubmitResult, TestsResult
+
+_PRECISION_ABS_REL_RE = re.compile(
+    r"(?:absolute|relative)\s+error[^.]*?10\s*[\^{]\s*\{?\s*[-\u2212]\s*(\d+)\s*\}?",
+    re.IGNORECASE,
+)
+_PRECISION_DECIMAL_RE = re.compile(
+    r"round(?:ed)?\s+to\s+(\d+)\s+decimal\s+place",
+    re.IGNORECASE,
+)
+
+
+def extract_precision(text: str) -> float | None:
+    m = _PRECISION_ABS_REL_RE.search(text)
+    if m:
+        return 10 ** -int(m.group(1))
+    m = _PRECISION_DECIMAL_RE.search(text)
+    if m:
+        return 10 ** -int(m.group(1))
+    return None
 
 
 class BaseScraper(ABC):
@@ -19,6 +42,9 @@ class BaseScraper(ABC):
     @abstractmethod
     async def stream_tests_for_category_async(self, category_id: str) -> None: ...
 
+    @abstractmethod
+    async def submit(self, contest_id: str, problem_id: str, source_code: str, language_id: str, credentials: dict[str, str]) -> SubmitResult: ...
+
     def _usage(self) -> str:
         name = self.platform_name
         return f"Usage: {name}.py metadata <id> | tests <id> | contests"
@@ -40,6 +66,9 @@ class BaseScraper(ABC):
     def _contests_error(self, msg: str) -> ContestListResult:
         return ContestListResult(success=False, error=msg)
 
+    def _submit_error(self, msg: str) -> SubmitResult:
+        return SubmitResult(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())
@@ -71,6 +100,21 @@ class BaseScraper(ABC):
                 print(result.model_dump_json())
                 return 0 if result.success else 1
 
+            case "submit":
+                if len(args) != 5:
+                    print(self._submit_error("Usage: <platform> submit <contest_id> <problem_id> <language_id>").model_dump_json())
+                    return 1
+                source_code = sys.stdin.read()
+                creds_raw = os.environ.get("CP_CREDENTIALS", "{}")
+                try:
+                    credentials = json.loads(creds_raw)
+                except json.JSONDecodeError:
+                    credentials = {}
+                language_id = get_language_id(self.platform_name, args[4]) or args[4]
+                result = await self.submit(args[2], args[3], source_code, language_id, credentials)
+                print(result.model_dump_json())
+                return 0 if result.success else 1
+
             case _:
                 print(
                     self._metadata_error(