[EPAC-1766]: centralize parliamentary topic taxonomy

[riddim-developer-bot]

Why

The parliamentary topic taxonomy had drifted between iOS and backend adapters. iOS already carried the fuller product-policy taxonomy, including the existing naturalresources topic and broader defence/justice keyword sets, while backend/topic-notifier used a stale copy. This PR moves topic IDs and keyword matching data behind one canonical repo source so topic follows, notifications, and search personalization all consume the same contract.

What changed

• Added shared/topic-taxonomy/parliamentary_topics.json as the canonical source of truth for topic IDs, backend display names, and keyword lists.
• Added scripts/topic_taxonomy/generate_topic_taxonomy.py to generate the Swift and Go adapters plus a --check mode that fails when generated outputs drift from the canonical taxonomy.
• Regenerated ios/epac/Model/ParliamentaryTopic.swift from the canonical taxonomy, preserving stable topic IDs for stored user preferences.
• Replaced the hand-copied backend topic list with generated Go code in backend/topic-notifier, keeping notification payload keys and topic preference semantics unchanged.
• Updated backend/search personalization to expand followed topic IDs into canonical keyword hints instead of guessing matches from the slug text alone.
• Recorded the explicit naturalresources decision in the canonical taxonomy metadata and added a minimal use-case catalog entry for FollowTopic, MatchParliamentaryTopics, and NotifyTopicFollowers.
• Added backend tests and expanded iOS topic-matching tests around the retained natural-resources topic and the broadened defence/justice keywords.

Trade-offs not taken

• I did not introduce runtime taxonomy file loading in either iOS or Lambda code. The adapters stay compile-time/generated so deployment packaging and app startup behavior remain unchanged.
• I kept backend notification copy/backing display names compatible with the prior backend strings where possible, since this issue is about data-source centralization rather than changing notification copy strategy.

Test plan

• python3 scripts/topic_taxonomy/generate_topic_taxonomy.py --check
• cd backend/topic-notifier && go test -coverprofile=coverage.out ./... && go tool cover -func=coverage.out
• cd backend/search && go test -coverprofile=coverage.out ./... && go tool cover -func=coverage.out
• swiftlint --strict
• python3 scripts/localization/check_localizations.py --github-warnings
• cd ios && xcodebuild -project epac.xcodeproj -scheme epac -configuration Debug -sdk iphonesimulator -destination 'platform=iOS Simulator,name=iPhone 17 Pro Max' -derivedDataPath .build build
• cd ios && xcodebuild test -project epac.xcodeproj -scheme epac -destination 'platform=iOS Simulator,name=iPhone 17 Pro Max' -derivedDataPath .build -enableCodeCoverage YES -only-testing:epacTests/NaturalResourcesStatisticsTests

Verification notes

• cd ios && make build could not be used because xcbeautify is not installed in this environment, so I ran the equivalent xcodebuild command directly.
• No UI screens changed, so no screenshots were required.

Resolves EPAC-1766
Reviewer-Boundary: review-only

[riddim-reviewer-bot]

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