Add Zod/Joi validation schemas for all API routes

[jobbykings]

Description

Many route handlers lack proper request body validation. Routes like assignmentRoutes.ts, collaborationRoutes.ts, and notificationRoutes.ts accept user input without schema validation. Implement Zod schemas and validation middleware for every route that accepts user input.

Files to Modify

• New: backend/src/utils/validation.ts — Shared validation utilities
• Each route file under backend/src/routes/ — Add schema validation
• backend/src/middleware/validateRequestSchema.ts — Enhance validation middleware

Definition of Done

• Zod schemas defined for every POST/PUT/PATCH request body
• Validation middleware applied to all routes accepting user input
• Descriptive validation error messages returned to client
• Query parameter validation for GET endpoints with filters
• No raw req.body access without prior validation
• Tests verify validation rejects malformed requests

[MerlinTheWhiz]

Hi, there. I can implement the API routes validation schemas nicely.
Please assign.

[grantfox-oss]

🦊 GrantFox — @MerlinTheWhiz has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #58)
2. Your PR will be reviewed by the Epondia maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@MerlinTheWhiz's PR #105 was approved and merged by @jobbykings.

🏆 @MerlinTheWhiz: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,058 total points). Track your full progress on GrantFox.

👏 Great work, @MerlinTheWhiz! Keep contributing to Epondia.