perf(pb,frontend): cascadeDelete bunk_assignments_draft on scenario delete

[adamflagg]

Summary

Follow-up to #992 (scoreboard item #16b). #992 fixed the visible "list vanishes" bug; this PR kills the underlying "long delete" — the several-second serial-delete loop that made the disappearance so noticeable.

Root cause: bunk_assignments_draft.scenario was declared with cascadeDelete: false in migration 1500000022, so useDeleteScenario had to fetch every draft assignment for the scenario and delete them one-by-one from the frontend before deleting the scenario itself. For a real session, that's N+1 round-trips against PocketBase on every delete.

Fix: New migration 1500000098 flips the relation to cascadeDelete: true (all other properties preserved exactly as in 1500000022). useDeleteScenario collapses to a single pb.collection('saved_scenarios').delete(id) — PocketBase cascades the draft rows server-side.

Base branch

This PR targets feature/fix-delete-scenario-render (PR #992) rather than main. Once #992 merges, I'll retarget this at main or it'll auto-rebase.

Test plan

• Failing TDD test first (useDeleteScenario asserted no calls to bunk_assignments_draft.getFullList / .delete — red against the N+1 loop).
• Implementation: single-call mutation; test goes green.
• Full frontend suite: 2797 passing.
• prettier / tsc / eslint (0 errors) clean; pre-push hooks green.
• Migration symmetry: down restores cascadeDelete: false so a rollback re-requires the frontend loop if paired with an older build.

Manual validation (dev/preview)

Scoreboard item covered: #16b — cascadeDelete bunk_assignments_draft (long-delete fix, follow-up to #16)

Requires PR #992 to be merged first, OR testing against the #994 branch directly.

• Open a session with a saved scenario containing many draft bunk assignments (ideally a populated session, not an empty one).
• Delete the scenario. Time it roughly.
• Expected: delete completes in ≲1 second (single server-side cascade).
• Compare: before this PR, delete was multi-second due to N+1 frontend round-trips deleting each draft assignment individually.
• Verify scenario is gone from the modal list and no orphan bunk_assignments_draft rows remain (spot-check via PocketBase admin or a query if available).
• Create a new scenario and confirm it still works end-to-end (migration didn't break upstream writes).

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: bab9b6d3-5b31-425e-8fab-edfb46ac4398

📥 Commits

Reviewing files that changed from the base of the PR and between 28d38df and 86e7630.

📒 Files selected for processing (9)

• .pip-audit-ignore
• frontend/src/components/ScenarioManagementModal.test.tsx
• frontend/src/components/ScenarioManagementModal.tsx
• frontend/src/contexts/ScenarioContext.test.tsx
• frontend/src/contexts/ScenarioContext.tsx
• frontend/src/hooks/useSavedScenariosMutation.test.ts
• frontend/src/hooks/useSavedScenariosMutation.ts
• frontend/src/hooks/useScenario.ts
• pocketbase/pb_migrations/1500000098_bunk_assignments_draft_scenario_cascade_delete.js

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/scenario-cascade-delete

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