fastapi: added post doc

Docs/Services/FastAPI/04-Post-Method.md

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+# FastAPI – POST Endpoint and JSON Input
+
+This section demonstrates how to handle **POST requests** in FastAPI to create new resources using request data and return JSON responses.
+
+---
+
+## Example Application (POST Request)
+
+Extend `main.py` with the following code:
+
+```python
+from fastapi import FastAPI
+
+app = FastAPI()
+
+users = [
+    {"name": "abbas", "age": 20},
+    {"name": "mmd", "age": 37},
+    {"name": "asghar", "age": 19},
+]
+
+
+@app.get("/")
+def home_page():
+    return {"msg": "API is working"}
+
+
+@app.post("/new_user")
+def create_user(name: str, age: int):
+    new_user = {"name": name, "age": age}
+    users.append(new_user)
+    return {"msg": "User created successfully"}
+```
+
+---
+
+## Code Overview
+
+### POST Endpoint Definition
+
+```python
+@app.post("/new_user")
+def create_user(name: str, age: int):
+```
+
+* Registers an HTTP **POST** endpoint at `/new_user`
+* Accepts input parameters:
+
+  * `name` → string
+  * `age` → integer
+* FastAPI automatically validates input types
+
+---
+
+### Creating a New Resource
+
+```python
+new_user = {"name": name, "age": age}
+users.append(new_user)
+```
+
+* Constructs a new user object
+* Appends it to the in-memory `users` list
+* Simulates creating a record in a database
+
+---
+
+### JSON Response
+
+```python
+return {"msg": "User created successfully"}
+```
+
+* Returns a structured JSON response
+* Automatically serialized by FastAPI
+
+---
+
+## Example Request
+
+### Using `curl`
+
+```bash
+curl -X POST "http://localhost:8000/new_user?name=ali&age=25"
+```
+
+### Response
+
+```json
+{
+  "msg": "User created successfully"
+}
+```
+
+---
+
+## Verifying the Result
+
+After creating a user, retrieve the updated list:
+
+```http
+GET /users
+```
+
+Response will now include the newly added user.
+
+---
+
+## Running the Application
+
+Start the FastAPI service using `uvicorn`:
+
+```bash
+uvicorn main:app --reload
+```
+
+---
+
+## Best Practices
+
+* POST requests should be used to create resources
+* Avoid modifying in-memory data in production environments
+* Use request bodies with Pydantic models instead of query parameters for real APIs
+* Return appropriate HTTP status codes (`201 Created`)
+* Validate and sanitize all client-provided input
+* Replace in-memory storage with persistent databases
+