Add bounded collector for dev app backend diagnostics

[lawrencecchen]

Land the independent collector from #13890 first so native app delivery can be verified while its macOS checks wait for a runner.

The fixed-schema route accepts only bounded development tag/revision, timing and outcome codes, applies the existing diagnostics rate limit before parsing, and acknowledges only after Axiom confirms the entire batch. The ingestion token stays server-side and the dataset is fixed. No authentication is required because the backend failure can itself prevent sign-in; records are untrusted operational observations, not account identities.

Validation: collector behavior tests, web typecheck and complexity gate passed. No database migration or existing endpoint changes. The server-only production ingestion credential is provisioned.

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Summary by cubic

Adds a bounded, unauthenticated collection endpoint so tagged DEBUG apps can report development backend connection outcomes even when a backend failure prevents sign-in.

• Accepts only a fixed versioned schema and applies the existing diagnostics rate limit before parsing.
• Acknowledges only after Axiom confirms the entire batch; the ingestion token stays server-side.
• Records are untrusted operational observations, not account identities.
• No database migration or existing endpoint changes.

^{Written for commit d9703b8. Summary will update on new commits.}

Summary by CodeRabbit

• New Features
  • Added diagnostic reporting for development backends, with validated event data and delivery acknowledgements.
  • On Vercel, diagnostic requests are subject to rate limits; requests are rejected if rate limiting is unavailable or not configured.
• Documentation
  • Documented diagnostic retention, retries, consent, delivery, and the debug.dev_backend.check command.

[github-actions]

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📒 Files selected for processing (4)

• web/app/api/observability/dev-backend/route.ts
• web/services/observability/DEV-BACKEND.md
• web/services/observability/devBackendDiagnostics.ts
• web/tests/dev-backend-diagnostics.test.ts

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