Update documentation for schema validation and constraints

- Add comprehensive schema constraints documentation to API.md
* Document all 5 constraint types: required, type, range, pattern, enum
* Include request/response examples for each constraint type
* Add practical examples showing constraint validation in action

- Update README.md to:
* Add schema validation to the features list
* Fix property format in example (use simple key-value instead of nested)

- Update embedded API documentation in rest.rs to include:
* POST /api/v1/constraints endpoint documentation
* GET /api/v1/constraints endpoint documentation
* Example constraint creation request/response

- Add detailed examples showing constraint workflow:
* Creating constraints for Employee nodes
* Demonstrating validation errors
* Successful node creation with constraints

Documentation now accurately reflects the complete schema validation system.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[e-mail]>

README.md

···
 - **🔐 Authentication**: JWT-based auth with role-based access control
 - **💾 Persistence**: Optional RocksDB backend for data durability
 - **📈 Monitoring**: Built-in metrics and observability
+- **✅ Schema Validation**: Comprehensive constraint system for data integrity
 
 ## 🚀 Quick Start
 
···
   -H "Content-Type: application/json" \
   -d '{
     "labels": ["Person"],
-    "properties": [
-      {"key": "name", "value": {"string_value": "Alice"}},
-      {"key": "age", "value": {"int_value": 30}}
-    ]
+    "properties": {
+      "name": "Alice",
+      "age": 30
+    }
   }'
 
 # Execute Cypher query

docs/API.md

···
 
 Returns this API documentation in JSON format.
 
+### Schema Constraints
+
+#### Create Constraint
+
+**POST** `/constraints`
+
+Creates a new schema constraint to enforce data integrity rules.
+
+**Request Body:**
+```json
+{
+  "constraint_type": "required",
+  "label": "Employee",
+  "property": "email"
+}
+```
+
+**Constraint Types:**
+
+1. **Required Constraint**
+```json
+{
+  "constraint_type": "required",
+  "label": "Employee",
+  "property": "email"
+}
+```
+
+2. **Type Constraint**
+```json
+{
+  "constraint_type": "type",
+  "label": "Employee",
+  "property": "salary",
+  "type_value": "integer"
+}
+```
+Supported types: `string`, `integer`, `float`, `boolean`
+
+3. **Range Constraint**
+```json
+{
+  "constraint_type": "range",
+  "label": "Employee",
+  "property": "salary",
+  "min_value": 30000,
+  "max_value": 200000
+}
+```
+
+4. **Pattern Constraint**
+```json
+{
+  "constraint_type": "pattern",
+  "label": "User",
+  "property": "email",
+  "pattern": "@"
+}
+```
+
+5. **Enum Constraint**
+```json
+{
+  "constraint_type": "enum",
+  "label": "Task",
+  "property": "status",
+  "allowed_values": ["pending", "in_progress", "completed"]
+}
+```
+
+**Response:**
+```json
+{
+  "id": "required:Employee:email",
+  "constraint_type": "required",
+  "label": "Employee",
+  "property": "email",
+  "details": {}
+}
+```
+
+#### List Constraints
+
+**GET** `/constraints`
+
+Lists all defined schema constraints.
+
+**Response:**
+```json
+[
+  {
+    "id": "required:Employee:email",
+    "constraint_type": "required",
+    "label": "Employee",
+    "property": "email",
+    "details": {}
+  },
+  {
+    "id": "type:Employee:salary",
+    "constraint_type": "type",
+    "label": "Employee",
+    "property": "salary",
+    "details": {
+      "type": "Integer"
+    }
+  }
+]
+```
+
 ## gRPC API
 
 The gRPC API provides the same functionality as the REST API but uses Protocol Buffers for serialization. The service is defined in `proto/gigabrain.proto`.
···
 # Delete a specific property
 curl -X DELETE http://localhost:3000/api/v1/nodes/1/properties/age \
   -H "Authorization: Bearer <token>"
+```
+
+### Working with Schema Constraints
+
+```bash
+# Create a required email constraint for Employee nodes
+curl -X POST http://localhost:3000/api/v1/constraints \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer <token>" \
+  -d '{
+    "constraint_type": "required",
+    "label": "Employee",
+    "property": "email"
+  }'
+
+# Create a type constraint for salary
+curl -X POST http://localhost:3000/api/v1/constraints \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer <token>" \
+  -d '{
+    "constraint_type": "type",
+    "label": "Employee",
+    "property": "salary",
+    "type_value": "integer"
+  }'
+
+# Create a range constraint for salary
+curl -X POST http://localhost:3000/api/v1/constraints \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer <token>" \
+  -d '{
+    "constraint_type": "range",
+    "label": "Employee",
+    "property": "salary",
+    "min_value": 30000,
+    "max_value": 200000
+  }'
+
+# Try to create an employee without email (will fail)
+curl -X POST http://localhost:3000/api/v1/nodes \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer <token>" \
+  -d '{
+    "labels": ["Employee"],
+    "properties": {
+      "name": "John Doe",
+      "salary": 50000
+    }
+  }'
+# Response: {"error": "Validation error: Required property 'email' is missing", "code": 400}
+
+# Create an employee with all required properties
+curl -X POST http://localhost:3000/api/v1/nodes \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer <token>" \
+  -d '{
+    "labels": ["Employee"],
+    "properties": {
+      "name": "John Doe",
+      "email": "[e-mail]",
+      "salary": 50000
+    }
+  }'
 ```
 
 ### Running Graph Algorithms

src/server/rest.rs

···
                     }
                 }
             },
+            "constraints": {
+                "POST /api/v1/constraints": {
+                    "description": "Create a schema constraint",
+                    "request_body": {
+                        "constraint_type": "required",
+                        "label": "Employee",
+                        "property": "email"
+                    },
+                    "response": {
+                        "id": "required:Employee:email",
+                        "constraint_type": "required",
+                        "label": "Employee",
+                        "property": "email",
+                        "details": {}
+                    }
+                },
+                "GET /api/v1/constraints": {
+                    "description": "List all schema constraints",
+                    "response": [
+                        {
+                            "id": "required:Employee:email",
+                            "constraint_type": "required",
+                            "label": "Employee",
+                            "property": "email",
+                            "details": {}
+                        }
+                    ]
+                }
+            },
             "docs": {
                 "GET /api/v1/docs": {
                     "description": "This API documentation",