feat(proto): add enums for mode fields in SummarizeArticle and GetPopulationExposure

[NewCoder3294]

Summary

• Add SummarizeMode enum (SUMMARIZE_MODE_UNSPECIFIED, SUMMARIZE_MODE_BRIEF, SUMMARIZE_MODE_ANALYSIS, SUMMARIZE_MODE_TRANSLATE) to summarize_article.proto and change SummarizeArticleRequest.mode from string to SummarizeMode
• Add PopulationExposureMode enum (POPULATION_EXPOSURE_MODE_UNSPECIFIED, POPULATION_EXPOSURE_MODE_COUNTRIES, POPULATION_EXPOSURE_MODE_EXPOSURE) to get_population_exposure.proto and change GetPopulationExposureRequest.mode from string to PopulationExposureMode
• Regenerate TypeScript client/server code and OpenAPI docs to reflect the new enum types
• Update all server handlers and client call sites to use enum values instead of raw string comparisons

Closes #192

Test plan

• Verify npx tsc --noEmit passes with zero errors
• Verify summarization flow works end-to-end (brief, analysis, translate modes)
• Verify population exposure endpoint works for both countries and exposure modes
• Verify cache key generation still produces consistent keys with the new enum values

🤖 Generated with Claude Code

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