feat: suppress access logs for Playwright test requests

[maxachis]

Summary

• Adds ASGI middleware that detects the X-Test-Run: true header sent by Playwright tests and suppresses their uvicorn access log entries
• Uses contextvars + logging.Filter for async-safe, zero-dependency log suppression
• Reduces Papertrail log volume from ~450MB/month to ~50MB/month

Closes #937

Changes

• New: middleware/access_log_filter.py — SuppressTestAccessLogMiddleware (ASGI), TestRequestAccessLogFilter (logging), install_test_request_log_filter() helper
• Modified: app.py — integrates the middleware and log filter into create_asgi_app()
• New: tests/middleware/test_access_log_filter.py — unit tests for filter, middleware, and install function

Test plan

• uv run basedpyright --level error — 0 errors
• uv run ruff check . — all checks passed
• Unit tests verify: normal requests pass, test requests suppressed, contextvar resets after request, non-HTTP scopes pass through, case-insensitive header matching
• CI pipeline (pytest, ruff, basedpyright, bandit)

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