Add response caching for public creator profiles with cache invalidation

[nanaf6203-bit]

Area: Caching
Priority: P2 — Medium

Description

Public profile pages are read-heavy but rarely change. Cache them for 5 minutes to reduce Postgres load by 90%+ under normal traffic.

Acceptance Criteria

• Add @UseInterceptors(CacheInterceptor) + @CacheTTL(300) to GET /profiles/:username and GET /profiles/:username/tipping-info
• Use Redis cache backend (shared across instances)
• On profile update (PUT /profiles/me, PATCH social-links, POST avatar), evict cache for that user
• Cache hit/miss metrics added to Prometheus (cache_hits_total, cache_misses_total)
• Document cache invalidation strategy in docs/CACHING.md
• Test: profile read returns from cache; update invalidates; next read returns fresh data

Technical Notes

• Use cache-manager with redis store
• Cache key: profile:<username>:tipping-info and profile:<username>

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. Thank you!

[grantfox-oss]

🦊 GrantFox — @Menjay7 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #68)
2. Your PR will be reviewed by the StellarTips maintainers

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