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.

lua/cp/setup.lua

@@ -130,7 +130,8 @@ local function start_tests(platform, contest_id, problems)
         ev.timeout_ms or 0,
         ev.memory_mb or 0,
         ev.interactive,
-        ev.multi_test
+        ev.multi_test,
+        ev.precision
       )
 
       local io_state = state.get_io_view_state()