feat(oauth): structured logging for invalid_scope rejections at the AS

Context

Per the OAuth scope drift RFC's pre-flight #4, PostHog's AS doesn't emit invalid_scope rejections to the standard log stream. They come back as HTTP 302 redirects with ?error=invalid_scope in the redirect URL (per RFC 6749 § 4.1.2.1), but nothing logs the rejection on our side.

This means:

• The RFC's success criterion ("zero invalid_scope over 30-day window") is unmeasurable today
• We've been counting incidents by Slack reports + GitHub PRs, which is a lower bound
• Carmen's case (#57509) was caught manually in review, not surfaced by alerting

Confirmed via query-logs against posthog-web-django: zero results for searchTerm invalid_scope over the past 24h, despite the in-flight #57509 PR being the resolution to a customer-reported invalid_scope event in the same window.

Change

In posthog/api/oauth/views.py, add structured logging at the point where the AS returns an invalid_scope redirect.

Two places to instrument depending on where DOT raises:

1. The OAuthAuthorizationView (or its parent in DOT) — when validate_authorization_request() raises InvalidScopeError
2. The exception handler that converts the OAuth error into the redirect URL

Add:

import structlog
import posthoganalytics
logger = structlog.get_logger(__name__)

# At the rejection point:
logger.warning(
    \"oauth.authorize.invalid_scope\",
    requested_scopes=requested_scopes_list,
    grantable_scopes=grantable_scopes_list,
    missing_scopes=list(set(requested_scopes_list) - set(grantable_scopes_list)),
    client_id=request.GET.get(\"client_id\", \"\"),
    user_agent=request.headers.get(\"User-Agent\", \"\")[:200],
)
posthoganalytics.capture_exception(
    Exception(\"OAuth invalid_scope rejection\"),
    properties={
        \"missing_scopes\": list(set(requested_scopes_list) - set(grantable_scopes_list)),
        \"client_id\": request.GET.get(\"client_id\", \"\"),
    },
)

Confirm the WARN-severity Loki query produces results within 7 days of deploy.

Acceptance

• A query like SELECT count() FROM logs WHERE service.name = 'posthog-web-django' AND severity_text = 'warn' AND body ILIKE '%oauth.authorize.invalid_scope%' produces non-zero results once a real rejection occurs
• The baseline metric for the RFC's success criterion is available within 7 days of this landing
• Capture in error tracking surfaces the spike if there's a regression

Why

You can't measure what you don't observe. The RFC commits to "zero invalid_scope over 30 days" as the success metric for the structural cleanup; this issue makes that measurable.

Tracking

Parent: #57524
Project: https://github.com/orgs/PostHog/projects/194
RFC: https://github.com/PostHog/requests-for-comments-internal/blob/matt/oauth-scope-drift-rfc/engineering/2026-05-04-oauth-scope-drift.md

Start in posthog/api/oauth/views.py at OAuthAuthorizationView and trace where validate_authorization_request() raises InvalidScopeError or where DOT converts the error into a redirect. Review the existing query-logs search and verify that a WARN query for oauth.authorize.invalid_scope returns a real rejection after deployment. Done means the structured log, error-tracking capture, and seven-day baseline are available.