Add structured output quality scoring

[nguyenduc071912]

Summary

Implements the bounty request for multi-dimensional quality scoring of structured outputs.

• Adds deterministic scoring for JSON, markdown, code, and plain text submissions
• Scores all five requested dimensions with the requested weights:
  • Completeness: 0.30
  • Format Compliance: 0.20
  • Coverage: 0.25
  • Clarity: 0.15
  • Validity: 0.10
• Returns the requested response shape: weighted_score, quality_rating, scores, feedback, and pass_threshold
• Adds /api/quality-score so the scorer is available through the API
• Adds unit tests including a 100-submission performance check under 10 seconds
• Documents the new endpoint and rubric options in the README

Fixes #1.

Tests

python -m unittest test_quality_scoring.py -v
python -m py_compile quality_scoring.py app.py
python -c "from app import app; print(app.title); print([route.path for route in app.routes if 'quality' in route.path])"

I also verified the endpoint with fastapi.testclient.TestClient; it returned HTTP 200 and the expected scorecard schema.