feat(cpex-scraper): add file logger for better debugging and tracing

[faizkhairi]

Summary

• Add a lightweight file logger (src/logger.ts) that writes timestamped lines to both stdout and a log file
• Log files are written to scrapers/cpex-scraper/logs/ with format cpex-YYYY-MM-DD.HH-mm-ss.log
• The logs directory is created automatically at runtime (already covered by root .gitignore)
• Update src/index.ts to use the file logger instead of the default console
• Zero new dependencies -- uses only Node.js built-in fs module
• Satisfies the existing Pick<Console, 'log'> interface, so scraper.ts is unchanged

Related Issue

Closes #4371

Test Plan

• Run pnpm --filter nusmods-cpex-scraper build and verify compilation succeeds
• Run the scraper and verify log file is created in logs/ directory
• Verify console output still shows timestamped log lines
• Verify existing unit tests still pass (pnpm --filter nusmods-cpex-scraper test)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.39%. Comparing base (988c6fd) to head (b1610aa).
⚠️ Report is 220 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4376      +/-   ##
==========================================
+ Coverage   54.52%   56.39%   +1.87%     
==========================================
  Files         274      308      +34     
  Lines        6076     6963     +887     
  Branches     1455     1680     +225     
==========================================
+ Hits         3313     3927     +614     
- Misses       2763     3036     +273     

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[faizkhairi]

Note on testing: I was unable to run the full CI suite locally (pnpm run ci) as I used a clone. Happy to add unit tests for logger.ts if desired, the main cases would be:

1. log() writes timestamped lines to both stdout and the file stream
2. close() resolves after the stream is fully flushed
3. Constructor creates the logs directory if it doesn't exist
4. Stream error handler prevents unhandled crashes

The existing scraper.test.ts tests are unaffected since FileLogger is only instantiated in index.ts, not in the testable scraper.ts.

Let me know if you'd like me to add these tests.