Phase 4.3: enable backend strict mode and close surfaced diagnostics

[rendall]

Summary

Implements Phase 4.3 (docs/plans/phase-4_3-checklist.md) by enabling backend/functions TypeScript strict mode and remediating surfaced strictness diagnostics with minimal behavior change.

What changed

• Enabled "strict": true in tsconfig.netlify.functions.json.
• Added strict-safe error helpers in src/lib/backend-utilities.ts:
  • isApiError
  • toApiError
  • toErrorBody
• Updated function handlers to narrow caught unknown errors before returning API responses:
  • src/functions/auth.ts
  • src/functions/comment.ts
  • src/functions/gauth.ts
  • src/functions/topic.ts
  • src/functions/verify.ts
• Updated src/lib/MongodbService.ts strict hotspots:
  • _client / _db optional initialization
  • unknown catch-body normalization for admin-facing 500 responses
  • verifyGET catch narrowing for TokenExpiredError
• Added backend test coverage for new error helpers in src/tests/backend/utilities.test.ts.
• Updated planning docs:
  • Marked all Phase 4.3 checklist items complete.
  • Recorded Phase 4.3 completion + Phase 4.4 gate decision.
  • Added Phase 4.4 checklist document.

Checklist status

Phase 4.3 checklist items C01-C06 are complete in docs/plans/phase-4_3-checklist.md.
No TODO(phase-04.4) suppressions were added.

Validation

Executed successfully:

• yarn run typecheck
• MONGOMS_DOWNLOAD_URL="https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-ubuntu2204-6.0.14.tgz" TZ="UTC" yarn test:backend
• TZ="UTC" yarn test:frontend
• yarn run ci:local

Notes:

• Existing lint warning remains in src/tests/backend/MongodbService.test.ts for an unused variable; no lint errors.
• Existing webpack warnings from mongodb bundling are unchanged and ci:local passes.

[Copilot]

Pull request overview

Enables TypeScript strict mode for Netlify backend/functions compilation and applies strict-safe error handling/narrowing across backend runtime paths, plus adds targeted tests and updates Phase 04 planning documentation.

Changes:

• Turned on "strict": true for backend/functions TypeScript builds.
• Introduced backend error helpers (isApiError, toApiError, toErrorBody) and updated handlers/service code to safely normalize unknown catch values.
• Added backend tests for the new error helpers and updated Phase 04 planning/checklist docs (including archiving Phase 4.2 checklist).

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tsconfig.netlify.functions.json	Enables TypeScript strict mode for backend/functions.
src/lib/backend-utilities.ts	Adds strict-safe error helper utilities used by handlers/services.
src/tests/backend/utilities.test.ts	Adds unit tests covering the new backend error helpers.
src/lib/MongodbService.ts	Adjusts strict hotspots (optional init fields; admin-safe error body normalization; TokenExpiredError narrowing).
src/functions/auth.ts	Normalizes caught unknown via toApiError before responding.
src/functions/comment.ts	Normalizes caught unknown via toApiError before responding.
src/functions/gauth.ts	Normalizes caught unknown via toApiError before responding.
src/functions/topic.ts	Normalizes caught unknown via toApiError before responding.
src/functions/verify.ts	Normalizes caught unknown via toApiError before responding (uses 400 fallback).
docs/plans/phase-4_3-checklist.md	Adds Phase 4.3 checklist document (strict baseline).
docs/plans/phase-4_4-checklist.md	Adds Phase 4.4 checklist document (env contract + runtime clarity).
docs/plans/phase-4_2_1-ci-local-parity-checklist.md	Removes Phase 4.2.1 checklist document.
docs/plans/phase-04-type-safety-and-env-handling.md	Updates Phase 04 plan status/scope and records phase completion + gate notes.
docs/archive/phase-4_2-checklist.md	Archives Phase 4.2 checklist document.

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