Fix Python execution order: move unregister endpoint after activities dict

[Copilot]

The unregister endpoint referenced the activities dictionary before it was defined, violating Python's execution order.

Changes

• Moved /activities/{activity_name}/unregister endpoint definition from line 24 (before activities dict) to line 113 (after signup endpoint)
• Added docstring for consistency with other endpoints

# Before: endpoint defined at line 24, activities dict at line 35
@app.post("/activities/{activity_name}/unregister")
async def unregister_from_activity(activity_name: str, email: str):
    if activity_name not in activities:  # NameError: activities not defined yet
        ...

# After: activities dict at line 25, endpoint at line 113
activities = { ... }

@app.post("/activities/{activity_name}/unregister")
async def unregister_from_activity(activity_name: str, email: str):
    if activity_name not in activities:  # Now references defined dict
        ...

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