Add Valkey datamodel backend

[Copilot]

Adds a new datamodel-valkey module implementing all five repository interfaces backed by Valkey (Redis-compatible) using Spring Data Redis with Lettuce.

New module: datamodel-valkey

• Repositories: ValkeyClientRepository, ValkeyAuthorizationRepository, ValkeyLoginStateRepository, ValkeyLoginCodeRepository, ValkeyJwkCacheRepository
• Storage model: Redis Hashes for entities, Sorted Sets for client pagination, Sets for authorization indexes, TTL via EXPIREAT for ephemeral records (login state, login code, JWK cache)
• Key schema:
  • sac:client:{clientId} / sac:clients (sorted set index)
  • sac:auth:{documentId} with subject/audience set indexes and lookup keys
  • sac:loginstate:{sessionId}, sac:logincode:{code}, sac:jwk:{url}:{kid} (all TTL-managed)
• Activation: Spring profile datamodel-valkey

Integration points

• server-manage/pom.xml, server-token/pom.xml — added datamodel-valkey as runtime dependency
• Parent pom.xml — added module to default build

Configuration

spring.profiles.active=datamodel-valkey
spring.data.redis.host=localhost
spring.data.redis.port=6379

Docs

• docs/modules/datamodel/valkey.md — module documentation with key schema and configuration reference
• Updated index.md and mkdocs.yml navigation

Validated against real Valkey 9

• Server-token: full client_credentials OAuth2 flow (token issuance, scope filtering, credential validation, JWT structure)
• Server-manage: startup and GraphQL auth guard
• Data persistence across restarts

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