Add DynamoDB state adapter

[anatolzak]

Closes #282

Summary

• Adds @chat-adapter/state-dynamodb — a serverless-native state adapter using DynamoDB single-table design
• Uses DynamoDBDocument client with shorthand methods (.put(), .get(), .delete(), .query(), .update(), .batchWrite())
• AWS SDK packages as peer dependencies to avoid version conflicts

Design

• Single table (pk/sk string keys, no GSIs) with all record types (subscriptions, locks, cache, lists)
• Atomic lock operations via DynamoDB conditional expressions
• Dual-layer TTL: DynamoDB native TTL (epoch seconds) for GC + app-level expiresAtMs checks for correctness
• List ordering via atomic counter (ADD seq :one) + 16-digit zero-padded sort keys
• Values stored as native DynamoDB types (no JSON serialization)

Test plan

• pnpm validate passes (typecheck, lint, knip, tests, build)
• 41 unit tests with mocked DynamoDBDocument client
• Manual test with DynamoDB Local + example Next.js app

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	@​aws-sdk/​client-dynamodb@​3.1014.0
	@​aws-sdk/​lib-dynamodb@​3.1014.0

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