Apply SOLID principles: repository layer, service action map, named constants

[Valamorde]

Summary

• Repository layer (src/repositories/): introduces connectionRepo, sceneRepo, and campaignRepo — plain objects that encapsulate all DynamoDB access patterns. Handlers no longer import DDB command types, key builders, or the table name directly.
• Services action map: mirrors the sendmessage dispatch pattern. The 8-branch if-chain in services/handler.ts becomes a Record<string, ServiceAction> map; each action moves to its own file under src/handlers/services/.
• Named constants (src/shared/constants.ts): eliminates magic numbers scattered across the codebase (TTL_SECONDS = 7200, DEFAULT_SCENE_ID = '666', BATCH_SIZE = 25, SCENE_ORDER_SPACING = 1_000_000).
• Discriminated union for VTTMessage in types.ts — handlers receive narrowed types.
• Code hygiene: removes console.log(event) in setCampaignData (was logging full requests including headers), replaces the profanity-laden error log in forward.ts with console.error, deduplicates getCurrentSceneId logic shared between dmjoin and playerjoin.

Notes

No observable behavior changes. Existing tests pass without modification — mockClient(DynamoDBDocumentClient) patches the shared singleton that all repositories use internally.

Test plan

• pnpm test — all tests pass
• No handler file imports from @aws-sdk/lib-dynamodb directly
• No handler file imports keys.ts directly
• No console.log with raw event data in production handlers