docs: Add REQUEST scope demos to FastAPI and Flask examples

[mikelane]

Summary

• Add RequestContext service with Scope.REQUEST to both FastAPI and Flask example apps
• Add /context endpoint demonstrating per-request unique IDs via request scoping
• Update example READMEs with request scoping documentation sections and API endpoint docs

Context

The middleware integration for request scoping (issue #339) is already fully implemented in python/dioxide/fastapi.py and python/dioxide/flask.py with 40 passing integration tests. The remaining gap was that example applications did not demonstrate Scope.REQUEST usage. This PR fills that gap.

The production code includes:

• DioxideMiddleware (FastAPI): ASGI middleware creating ScopedContainer per request
• configure_dioxide() (Flask): Sets up before_request/teardown_request hooks for scope lifecycle
• Inject() and inject(): Resolve from the request scope automatically
• ScopedContainer: Caches REQUEST-scoped instances per scope, disposes lifecycle components on exit

Test plan

• All 40 existing FastAPI/Flask integration tests pass
• Linting passes on all changed files (ruff check)
• Formatting passes (ruff format --check)
• Pre-commit hooks pass

Fixes #339

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (9f37e0b) to head (1e8d1c3).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

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  Coverage   91.85%   91.85%           
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  Files          21       21           
  Lines        1903     1903           
  Branches      372      372           
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  Hits         1748     1748           
  Misses         91       91           
  Partials       64       64           

Flag	Coverage Δ
python-3.13	91.85% <ø> (ø)

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