[Bug] Alias-chain integrity: app-level cycle/target validation; destinationType unused

[Salem874]

Problem

The alias chain (tblShortURLs.redirectAlias) has no FK and no true cycle detection — it relies solely on the 3-hop counter in sp_lookupShortURL. destinationType is never read by the resolver.

Fix

Add app-level cycle/target-exists validation on alias create/edit; make destinationType authoritative in the resolver or drop it.

Acceptance criteria

• Cannot save an alias that targets a non-existent code or forms a cycle.
• destinationType is either used or removed.

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From the 2026-06-04 schema review — see docs/SCHEMA_REVIEW_2026-06-04.md.