refactor(frontend): readonly on app-types cache/API shapes

[adamflagg]

Backlog row §3 #8, second/final slice — types/app-types.ts (graph.ts shipped in #1592). This closes out #8.

What it does

Adds readonly to every field of the hand-written app-types interfaces — Camper, BunkRequest, Constraint, SolverRun, DragItem, BunkWithCampers — with readonly T[] on array fields and Readonly<Record<…>> on map fields (matching the nested-Record pattern #1592 picked up from CodeRabbit). Pure type-level — no runtime change.

The generated PB aliases (Session, Bunk, SavedScenario) are left mutable — their immutability belongs in codegen, not a hand-edit.

Why

Camper/BunkRequest are the domain shapes that live in the React Query cache and flow through the board, panels, and drag-and-drop. A stray in-place mutation on a shared cached instance would corrupt every subscriber; readonly makes that a compile error.

Cascade — measured, tiny

A full-readonly tsc probe surfaced exactly 2 consumer sites (even with Camper used in 105 files), both the predicted readonly T[]→mutable-param class, neither a real mutation:

Site	Fix
csvExportHelpers.ts buildCamperRows(campers)	widened param to readonly Camper[] — it only filter/toSorted/maps (all return fresh arrays)
RequestForm.tsx useState(constraint?.campers ?? [])	copies into a mutable working copy [...]; source stays immutable

Verification

• npm run type-check (both tsconfigs): green
• csvExportHelpers vitest: 19/19
• full lefthook run pre-push: green

Consistent with #1592's finding: zero in-place mutations of cached data exist today, so readonly is forward-looking insurance with near-zero cascade. Marks §3c #8 done (both slices); next live row is #9 (React Query queryOptions/skipToken).

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Fixed potential issue in request form where camper selections could inadvertently modify source data.

• Documentation

  • Updated modernization backlog to reflect completion of type system improvements.

• Refactor

  • Improved internal data immutability across core application types to enhance code reliability and prevent unintended data mutations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: c95a311e-f8bd-462e-9f1b-3082d064904a

📥 Commits

Reviewing files that changed from the base of the PR and between 1970b93 and 758c49c.

📒 Files selected for processing (4)

• docs/reference/modernization-backlog.md
• frontend/src/components/RequestForm.tsx
• frontend/src/types/app-types.ts
• frontend/src/utils/csvExportHelpers.ts

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📝 Walkthrough

Walkthrough

Readonly immutability is added to six core frontend type interfaces (Camper, BunkRequest, Constraint, SolverRun, DragItem, BunkWithCampers), including deep nested expand and results fields. csvExportHelpers tightens its API contract to accept readonly arrays, and RequestForm adapts by cloning the readonly source. Backlog documentation is updated to mark #1592 complete.

Changes

Type Immutability and API Contracts

Layer / File(s)	Summary
Core type immutability in app-types.ts frontend/src/types/app-types.ts	File-level comment explains hand-written shapes use readonly (arrays and Readonly<Record>). Camper, BunkRequest, Constraint, SolverRun, DragItem, and BunkWithCampers interfaces add readonly to all fields, including nested expand and results structures, and convert mutable arrays/objects to readonly equivalents.
API contract tightening in csvExportHelpers frontend/src/utils/csvExportHelpers.ts	buildCamperRows parameter updated from Camper[] to readonly Camper[].
RequestForm component adaptation frontend/src/components/RequestForm.tsx	selectedCampers state initialization uses array spread to clone constraint.campers, with comment clarifying the local copy is mutable while the source is readonly.
Modernization backlog documentation docs/reference/modernization-backlog.md	#1592 row updated to reflect both graph.ts and app-types.ts slices completed in this PR instead of app-types remaining open.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related issues

• refactor(frontend): seal readonly leak in createGraphElements cast (SocialNetworkGraph) #1593: Prevents unsafe type-casting that would strip readonly modifiers from graph node/edge data in SocialNetworkGraph/createGraphElements, a follow-up to readonly enforcement.

Possibly related PRs

• adamflagg/kindred#1587: Updates buildCamperRows in csvExportHelpers to use non-mutating Array.prototype.toSorted(); paired with the readonly parameter contract change here.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/readonly-app-types

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