Convert string-typed enum fields to proper proto enums

[SebastienMelki]

From PR #106 review — item M-10

Several request fields use bare strings where proper proto enums would be more appropriate:

• GetPopulationExposureRequest.mode: accepts "countries" or "exposure"
• SummarizeArticleRequest.mode: accepts "brief", "analysis", or "translate"
• SearchGdeltDocumentsRequest.timespan: accepts "15min", "1h", "24h"

Problem: Bare strings provide no compile-time validation, no autocomplete, and no documentation of valid values. Clients have to guess or read handler source code.

Suggested fix: Define proper proto enum types for each and update handlers to accept enum values.

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Deferred from PR #106 code review

[SebastienMelki]

Deferring — converting string-typed fields to proper proto enums affects multiple protos and their corresponding handler implementations. While enums would be better for type safety, the current string approach works and the refactor scope is significant. Will batch this with other proto schema improvements.