fix(register): Decision schema union types → OR-supported nullable (Decision couldn't re-import)

[rubenvdlinde]

Bug

The `Decision` schema has 7 properties (`sourceApp`, `subjectRegister`, `subjectSchema`, `subjectId`, `subjectLabel`, `outcomeCallbackUrl`, `externalReference`) declared with JSON-Schema union type `["string","null"]`. OpenRegister's schema validator rejects array types:

```
Failed to import schema: Invalid type '["string","null"]' at '/sourceApp'.
Must be one of: string, number, integer, boolean, array, object, null, file, geo, color, recurrence, NcFile, …
```

So the `Decision` schema fails every register (re)import — it is frozen in the DB at its 2026-06-26 state, and a fresh install cannot create it at all. Discovered live while applying the toezeggingen change: the decidesk register import silently dropped Decision on every run (Pass-1 import failure, caught-and-continued).

Fix

Convert each to the OR-supported nullable form already used elsewhere in this same register (`DecisionStage.outcome` / `.decidedAt` / `.note`):

```diff

• "type": ["string", "null"],

• "type": "string",
• "nullable": true,
  ```

Verification

• JSON valid; 0 union-typed properties remain.
• Live-verified on 8080: after this fix, `Decision` no longer appears in the import's Pass-1 failures (the only remaining failures are unrelated other-app schemas: `brpPerson`, `mandaat`).
• Scope: decidesk-only — 0 union-typed properties in openregister / doriath / hrmq / portaliq / docudesk / opencatalogi, so this is not a fleet-wide OR-validator regression.

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