Consolidate validation on Zod (replace AJV, Joi, and bespoke validators)

[carlgieringer]

Remaining:

• Consolidate API entities errors on IssueFormat. #268
• Replace AJV validation schemas with Zod #269
• Replace Joi validators with Zod #270
• Replace bespoke validators with Zod #271

Related: figure out our API error response.

Now it's like:

{proposition: BespokeValidationErrors<Proposition>}

Perhaps we could do this instead:

• z.ZodError<typeof Request> or
• {error: z.ZodError<typeof Request>}