Revert Autonomous Fix

Frederick's own described interaction, verbatim (2026-08-09): "if I want to undo it, I could even go to that issue and revert it, click a button that says revert -- the fix he just put -- or if I'm fine with it, I can leave it alone." Doing nothing is the default -- this route is the ONLY way an autonomous fix (app/modules/audit/railway_autofix.py) is ever undone; nothing in this codebase calls it unattended. Called from the issue detail screen's own "Revert this fix" card (apps/web/src/pages/issues/IssueDetailPage.tsx), drawn only for an issue that actually carries an autonomous fix and only for someone holding the grant this route requires.

Gated on issues:approve_production -- the decorator (REVERT_AUTOFIX, module-level, see this module's own docstring) AND, redundantly, railway_autofix.revert_fix itself (RevertNotPermitted) -- the same grant that gates a production deploy, because undoing a production infrastructure change is exactly as consequential as approving one.