[FEATURE] Add Zod schema validation for all request bodies #575

Category: backend / security
Project area: backend/src/middleware/

Context:
Validation is partially implemented. Full coverage needed.

Implementation:

• Zod schemas for: create campaign, pledge, reconcile, claim, refund, update metadata.
• Middleware returns 422 with field-level errors.
• Schemas co-located with route files.

Acceptance Criteria:

• All write endpoints validated.
• Error response includes field path and message.
• Invalid requests never reach route handler.