fix(databases): update default Redis version from 7 to 8

[GuillaumeLecomte1]

Summary

Update Redis default image from redis:7 to redis:8 in:

• packages/server/src/setup/redis-setup.ts (internal Redis setup)
• packages/server/src/services/ai.ts (AI service template examples)
• apps/dokploy/components/dashboard/project/add-database.tsx (UI default)

Redis 8 provides better performance according to benchmarks while maintaining stability.

Closes #4172

Greptile Summary

This PR bumps the default Redis image from redis:7 to redis:8 across the UI placeholder, AI prompt template, and the internal Dokploy Redis setup. Redis 8 has been GA since May 2025 and is available on Docker Hub. The db schema (packages/server/src/db/schema/redis.ts) was already at redis:8 before this PR, so the three changes here bring the rest of the codebase in sync — no redis:7 references remain.

Confidence Score: 5/5

• Safe to merge — clean version bump with no remaining redis:7 references and no logic changes.
• All three changes are straightforward string replacements. Redis 8 is confirmed GA and available on Docker Hub. The db schema was already at redis:8 before this PR, so these changes are consistent. No functional logic is altered and no redis:7 references remain in the codebase.
• No files require special attention.

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