Fix /v1/internal/backfill-stale 500 from mixed-type Map

[rainxchzed]

Summary

PR #9 deployed and the backfill endpoint immediately 500'd:

```
java.lang.IllegalStateException: Serializing collections of different element
types is not yet supported. Selected serializers: [kotlin.Int, kotlin.Boolean]
at io.ktor.serialization.kotlinx.SerializerLookupKt.elementSerializer
...
at zed.rainxch.githubstore.routes.InternalRoutesKt$internalRoutes$2$2.invokeSuspend(InternalRoutes.kt:526)
```

Root cause: `mapOf("scheduled" to N, "started" to true)` is a `Map<String, Any>` and kotlinx.serialization can't infer a single element serializer when values span `Int` + `Boolean` + `String`. Same for the no-candidates and conflict branches.

Fix

Replace the three response `mapOf` calls with a typed `@Serializable BackfillResponse` DTO:

```kotlin
@serializable
data class BackfillResponse(
val scheduled: Int,
val started: Boolean,
val message: String? = null,
)
```

Three callers:

• 202 `Accepted` after a successful schedule: `scheduled=N, started=true`
• 200 when nothing to backfill: `scheduled=0, started=false, message="no stale rows"`
• 409 when another backfill is already running: `scheduled=0, started=false, message="backfill_already_running"`

Test plan

• `./gradlew test` -- green.
• After deploy, retry the backfill via SSH:
  ```
  docker exec github-store-backend-app-1 curl -sS -X POST \
  -H "X-Admin-Token: $ADMIN_TOKEN" \
  http://localhost:8080/v1/internal/backfill-stale
  ```
  Expect: 202 + `{"scheduled": , "started": true, "message": null}`. Watch `docker logs` for "Backfill done: ok=N gone=M ...".

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Reviewing files that changed from the base of the PR and between 8e24fa6 and ed31274.

📒 Files selected for processing (1)

• src/main/kotlin/zed/rainxch/githubstore/routes/InternalRoutes.kt

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-backfill-response

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