Backend: test data factory, accessibility prefs, bandwidth-adaptive payload, search scorer

[mftee]

Adds four small, self-contained utility modules under src/common/utils/:

• test-data-factory.ts — generic factory helper to reduce test fixture boilerplate
• accessibility-preferences.ts — validates and merges player accessibility preferences (contrast, TTS, font scale, color-blind mode)
• bandwidth-adaptive-payload.ts — selects a mobile response payload variant based on reported network type
• search-relevance-scorer.ts — scores and ranks search results by term-frequency overlap with the query

Closes #416
Closes #419
Closes #420
Closes #421

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