Add KDoc to public API across all modules (KOJAK-36)

[endrju19]

Summary

• Add KDoc documentation to all 15 undocumented public Kotlin files across 5 modules (core, http, kafka, postgres, mysql)
• Follow existing project KDoc style: concise, [ClassName] references, no @param/@return
• Documentation-only changes — zero code modifications, 80 lines added

Modules covered

Module	Files	What was documented
okapi-core	6	Data classes, enums, sealed interface (OutboxMessage, OutboxId, OutboxStatus, OutboxEntry, DeliveryResult, RetryPolicy)
okapi-http	4	Transport impl, delivery info, DSL builder with usage example, enum
okapi-kafka	3	Transport impl, delivery info, DSL builder with usage example
okapi-postgres	1	Store implementation class
okapi-mysql	1	Store implementation class

Test plan

• ./gradlew ktlintCheck — passed
• ./gradlew compileKotlin — passed (verifies KDoc [references] don't break compilation)
• No code changes — only KDoc comment additions