Evaluate service layer refactoring for links/redirect.rs

[piffio]

Context

src/api/links/redirect.rs is the hot path for all link redirects and currently
makes direct KV/repository calls, bypassing the service layer. Intentionally
excluded from the service layer migration (refactor/fix-backend-layer-violations)
due to performance concerns.

Current pattern

1. KV lookup for short code (sub-10ms edge read)
2. D1 fallback on KV miss
3. 301 redirect issued immediately
4. Async analytics write to D1 (fire-and-forget, post-response)

Question to evaluate

Can LinkService (or a new RedirectService) be introduced without adding latency?

Options:

• Thin service wrapper — same direct KV/D1 calls internally
• Partial refactoring — service for analytics write, direct for KV lookup
• Leave as-is — document the direct-call exception in architecture notes

Acceptance criteria

• Redirect latency must not regress (measure p50/p99 before and after)
• Code compiles cleanly and all integration tests pass
• If refactored: zero direct crate::repositories:: or crate::kv:: calls in handler****