feat: standard error envelope, OpenAPI generation, cursor pagination, and validation middleware

[Gbemi-programmer]

Closes #1785
Closes #1784
Closes #1787
Closes #1786

Changes

#1785 Standard error envelope

• DomainError (backend/services/shared/errors.ts) now builds one consistent shape (code, message, statusCode, details, requestId, userMessage, recovery). It still accepts the old calling styles, so existing callers keep working.
• HTTP status is taken from ERROR_HTTP_STATUS_MAP in apiResponse.ts when only an error code is given.
• The error codes are documented in docs/ERROR_HANDLING.md.

#1784 OpenAPI generation

• New scripts/generate-openapi.ts builds spec/openapi.yaml, developer-portal/docs/openapi.json and sdks/generated/endpoints.json from the source schemas.
• New scripts: openapi:generate and openapi:check. CI runs openapi:check and fails on drift.
• The spec is published at GET /api/docs/openapi.json, with a Swagger UI at GET /api/docs.

#1787 Cursor pagination

• List endpoints (plans controller and public API router) use cursor pagination with stable ordering and a bounded default page size.
• Requests without a cursor still work, so existing clients are unaffected.

#1786 Request validation middleware

• validationMiddleware.ts checks requests against a schema at the edge and rejects bad input with the standard error envelope (VALIDATION_ERROR plus details for each field).
• It is exported from backend/shared/middleware and wired into createApiServer. Documented in docs/VALIDATION.md.

Tests

• New tests: validationMiddleware.test.ts, generateOpenApi.test.ts.
• errors, apiResponse and pagination suites pass.
• pnpm run openapi:check reports no drift.

Notes

In backend/services/shared, 20 suites (50 tests) already fail on main. They fail exactly the same way on this branch, so this PR adds no new failures.

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