Fix duplicate student registration bug and add 6 new extracurricular activities

[Copilot]

This PR addresses the duplicate registration bug in the Mergington High School Activities system and expands the available extracurricular activities as part of the GitHub Copilot exercise.

Bug Fix: Duplicate Registration Prevention

The original signup_for_activity function allowed students to register multiple times for the same activity, creating duplicate entries in the participants list. This has been fixed by adding validation:

# Validate student is not already signed up
if email in activity["participants"]:
    raise HTTPException(status_code=400, detail="Student is already signed up")

Before: Students could register multiple times, creating duplicates like:

"participants": ["student@mergington.edu", "student@mergington.edu"]

After: Duplicate registrations return HTTP 400 with clear error message.

New Activities Added

Expanded the activities database from 3 to 9 activities, adding variety across different interest areas:

Sports Activities (2 new)

• Basketball Team - Competitive basketball training and games
• Swimming Club - Swimming training and water sports

Artistic Activities (2 new)

• Art Studio - Express creativity through painting and drawing
• Drama Club - Theater arts and performance training

Intellectual Activities (2 new)

• Debate Team - Learn public speaking and argumentation skills
• Science Club - Hands-on experiments and scientific exploration

All new activities include realistic schedules, participant limits, and empty participant lists ready for signups.

Testing

• ✅ Duplicate registration prevention works via both API and web interface
• ✅ All 9 activities display correctly in the web interface
• ✅ New activities accept signups successfully
• ✅ Application runs without errors

Fixes #1.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

@heathermathies-accesso 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.