[nycomp]

Parent: nyjc-computing/campus#816 (item 2 — client apps as trace producers). Refs #37.

What

Make classroom a campus audit trace producer: when AUDIT_TRACING_ENABLED=1, the app factory wires campus.audit.middleware.init_app into the Flask app, so every classroom request is recorded as a span in the audit service.

Combined with SDK trace propagation (nyjc-computing/campus-api-python#93), campus SDK calls made while handling a classroom request carry X-Request-ID/X-Parent-Span-ID and land as child spans — the audit waterfall then answers which classroom route fired each campus call, in what order (the duplicate GET /auth/v1/sessions/... question in campus#816).

Decisions (the open questions in #37)

• Dependency shape: resolved — no extraction needed. classroom already depends on campus-suite (pyproject, branch weekly) and the middleware ships in it, so wiring is imports + config only. Flagged on campus#816; extraction into a shared package stays a non-goal unless the contract grows (W3C traceparent).
• Config surface: opt-in (AUDIT_TRACING_ENABLED=1 + AUDIT_API_KEY=<producer key>), default off — a client app shouldn't ingest spans without an explicit key. Read once at app creation: flipping the flag is a redeploy, not a runtime change (same semantics as the campus services' gate). Dev deployment only for now; staging/prod get it when they exist.
• Volume: span ingestion is one async worker per service, fail-safe (missing/broken key only loses spans, never requests) — fine at classroom's scale.

Verification

• Local: middleware registers start_span/end_span with the flag set; X-Request-ID echoed on responses; flag unset → no hooks (kill-switch path); cross-repo contract check — the middleware's flask.g stash read by the new campus_python.tracing produces both headers.
• E2E (post-merge, on the dev deployment): classroom login should produce classroom root spans with auth calls as child spans; kill switch re-verified live with AUDIT_TRACING_ENABLED=0.

Notes

• poetry.lock bump for the SDK propagation commit lands as a follow-up commit on this branch once campus-api-python#93 merges (git dep pinned by lock).
• Pre-existing ruff findings in the touched file (I001/E722/E501) are unchanged by this PR.
• Follow-up (not this PR): classroom spans could carry the signed-in user (flask.g.user → span user_id) — needs a small stash in flask_campus.OAuthLoginManager (campus repo).