Enhance API documentation and validation using Zod

- Added automatic validation with Zod for Fastify routes, eliminating manual validation.
- Updated API documentation for `/api/db/status`, `/api/db/setup`, and user registration endpoints to include detailed descriptions, request bodies, and response schemas.
- Implemented Zod schemas for request and response validation in `loginEmail`, `registerEmail`, and `dbSetup` routes.
- Improved error handling and response structures for better clarity and consistency.
- Removed redundant manual validation logic from route handlers, relying on Fastify's built-in validation.

Author: JasonKrik

services/backend/API_DOCUMENTATION.md

@@ -90,14 +90,19 @@ To add OpenAPI documentation to your routes, define your request body and respon
 
 Make sure you have `zod` and `zod-to-json-schema` installed in your backend service.
 
+### Recommended Approach: Automatic Validation with Zod
+
+The power of Zod lies in providing **automatic validation** through Fastify's schema system. This approach eliminates manual validation and leverages Zod's full validation capabilities.
+
 ```typescript
 import { z } from 'zod';
 import { zodToJsonSchema } from 'zod-to-json-schema';
 
 // 1. Define your Zod schemas for request body, responses, etc.
 const myRequestBodySchema = z.object({
   name: z.string().min(3).describe("The name of the item (min 3 chars)"),
-  count: z.number().positive().describe("How many items (must be positive)")
+  count: z.number().positive().describe("How many items (must be positive)"),
+  type: z.enum(['mysql', 'sqlite']).describe("Database engine type")
 });
 
 const mySuccessResponseSchema = z.object({
@@ -134,24 +139,91 @@ const routeSchema = {
   }
 };
 
-// 3. Use the schema in your Fastify route definition
-fastify.post('/your-route', { schema: routeSchema }, async (request, reply) => {
-  // Your route handler logic here
-  // Fastify will automatically validate request.body against myRequestBodySchema
-  
-  // Example of returning a success response:
-  // return reply.status(200).send({ 
-  //   success: true, 
-  //   itemId: 'some-uuid-v4-here', 
-  //   message: 'Item processed successfully.' 
-  // });
-
-  // Example of returning an error response:
-  // return reply.status(400).send({ success: false, error: "Invalid name provided." });
-});
+// 3. Use the schema in your Fastify route definition with proper TypeScript typing
+interface RequestBody {
+  name: string;
+  count: number;
+  type: 'mysql' | 'sqlite';
+}
+
+fastify.post<{ Body: RequestBody }>(
+  '/your-route', 
+  { schema: routeSchema }, 
+  async (request, reply) => {
+    // ✅ Fastify has already validated request.body using our Zod schema
+    // ✅ If we reach here, request.body is guaranteed to be valid
+    // ✅ No manual validation needed!
+    
+    const { name, count, type } = request.body; // Fully typed and validated
+    
+    // Your route handler logic here
+    return reply.status(200).send({ 
+      success: true, 
+      itemId: 'some-uuid-v4-here', 
+      message: `Item ${name} processed successfully with ${count} items using ${type}.` 
+    });
+  }
+);
+```
+
+### Key Benefits of This Approach
+
+1. **Single Source of Truth**: Zod schemas define both validation AND documentation
+2. **Automatic Validation**: Fastify automatically validates requests before your handler runs
+3. **No Manual Validation**: Remove all manual `zod.parse()` calls and field checks
+4. **Better Error Messages**: Fastify provides detailed validation errors automatically
+5. **Type Safety**: Handlers receive properly typed, validated data
+6. **Cleaner Code**: No redundant validation logic in handlers
+
+### What NOT to Do (Anti-patterns)
+
+❌ **Don't do manual validation in handlers:**
+```typescript
+// BAD: Manual validation (redundant)
+const parsedBody = myRequestBodySchema.safeParse(request.body);
+if (!parsedBody.success) {
+  return reply.status(400).send({ error: 'Invalid request body' });
+}
+
+// BAD: Manual field checks (redundant)
+if (!request.body.name || !request.body.count) {
+  return reply.status(400).send({ error: 'Required fields missing' });
+}
+
+// BAD: Manual enum validation (redundant)
+if (request.body.type !== 'mysql' && request.body.type !== 'sqlite') {
+  return reply.status(400).send({ error: 'Invalid database type' });
+}
 ```
 
-**Note**: Older examples in this document (like the "Logout Route Documentation" below) might still show manually crafted JSON schemas. The recommended approach is now to use Zod with `zod-to-json-schema` as shown above for better type safety and maintainability.
+✅ **Do trust Fastify's automatic validation:**
+```typescript
+// GOOD: Trust the validation - if handler runs, data is valid
+const { name, count, type } = request.body; // Already validated by Fastify
+```
+
+### Validation Flow
+
+The validation chain works as follows:
+
+**Zod Schema → JSON Schema → Fastify Validation → Handler**
+
+1. **Zod Schema**: Define validation rules using Zod
+2. **JSON Schema**: Convert to OpenAPI format using `zodToJsonSchema()`
+3. **Fastify Validation**: Fastify automatically validates incoming requests
+4. **Handler**: Receives validated, typed data
+
+If validation fails, Fastify automatically returns a 400 error **before** your handler runs.
+
+### Real-World Examples
+
+See these files for complete examples of proper Zod validation:
+- `src/routes/db/setup.ts` - Database setup with enum validation
+- `src/routes/db/status.ts` - Simple GET endpoint with response schemas
+- `src/routes/auth/loginEmail.ts` - Login with required string fields
+- `src/routes/auth/registerEmail.ts` - Registration with complex validation rules
+
+**Note**: Older examples in this document (like the "Logout Route Documentation" below) might still show manually crafted JSON schemas. The recommended approach is now to use Zod with automatic Fastify validation as shown above.
 
 ## Example: Logout Route Documentation