refactor(proto): introduce shared ServiceError type and begin unifying error patterns

[NewCoder3294]

Summary

Closes #191 (first step).

The codebase currently has four different error patterns coexisting across proto response messages:

1. string error -- a plain string field (e.g. GetTemporalBaselineResponse, SearchGdeltDocumentsResponse, GetUSNIFleetReportResponse)
2. bool success + string error -- a success flag paired with an error string (e.g. ListTechEventsResponse, GetPopulationExposureResponse)
3. GeneralError -- a structured oneof for infrastructure-level errors like rate limiting, geo blocking, upstream outages (core/v1/general_error.proto, currently unused in any response message)
4. Gateway-level JSON errors -- { error: string } / { message: string } returned directly by the gateway error boundary and error-mapper

This PR introduces a shared ServiceError message type as the standard pattern for per-RPC business logic errors and migrates three response messages as a proof of concept:

• New: proto/worldmonitor/core/v1/service_error.proto with code (machine-readable) + message (human-readable) fields
• Migrated: GetTemporalBaselineResponse, RecordBaselineSnapshotResponse, SearchGdeltDocumentsResponse
• Old string error field numbers are reserved to maintain wire compatibility
• Handler code updated to construct { code, message } objects (e.g. INVALID_ARGUMENT, INTERNAL, UPSTREAM_ERROR)
• Client-side consumers (gdelt-intel.ts) updated to use resp.serviceError

Migration approach

The field is named service_error (not error) to avoid conflict with the reserved old field name. Future PRs will migrate the remaining response messages following this same pattern. The GeneralError type remains available for infrastructure-level cross-cutting concerns (rate limiting, geo blocking, etc.).

Changed files

File	Change
proto/worldmonitor/core/v1/service_error.proto	New shared ServiceError message
proto/.../get_temporal_baseline.proto	Replace string error with ServiceError service_error
proto/.../record_baseline_snapshot.proto	Replace string error with ServiceError service_error
proto/.../search_gdelt_documents.proto	Replace string error with ServiceError service_error
server/.../get-temporal-baseline.ts	Use serviceError: { code, message }
server/.../record-baseline-snapshot.ts	Use serviceError: { code, message }
server/.../search-gdelt-documents.ts	Use serviceError: { code, message }
src/services/gdelt-intel.ts	Update client references from resp.error to resp.serviceError
src/generated/...	Regenerated (infrastructure + intelligence only)
docs/api/...	Regenerated OpenAPI specs

Test plan

• tsc --noEmit passes (non-generated code has zero errors)
• buf lint passes for all modified proto files
• Verify GDELT search endpoint returns serviceError object on failure
• Verify temporal baseline endpoint returns serviceError on invalid params
• Verify record baseline snapshot endpoint returns serviceError on empty updates

🤖 Generated with Claude Code

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