Add strict input validation for predictions queries

[code-0-stella]

Add strict input validation for predictions queries

PR Description

This change adds route-boundary validation for GET /api/predictions using a dedicated Zod schema.

What changed

• Introduced a dedicated validator for predictions query parameters in predictions.ts
• Wired the predictions route to reject unexpected query params and enforce the existing validation rules in predictions.ts
• Added a regression integration test covering invalid/unexpected query parameters in predictions.test.ts

Why

This makes predictions input handling consistent with the project’s other validated routes and ensures malformed requests fail fast with the standard 400 validation error envelope.

Verification

• Ran ESLint on the touched files

• Ran the predictions integration suite:

  • 1/1 suite passed
  • 17/17 tests passed

  Closes: Add input validation for predictions (buffer #14) #350

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[greatest0fallt1me]

Merged into main via admin resolver (-X theirs).

[greatest0fallt1me]

LGTM 👌 merging. Solid contribution.