Update activity.py with GET endpoints

[guntejsingh27]

Implemented two GET routes: fetch all activities and fetch activity by ID.
Added GET endpoints in activity.py to fetch all activities and single activity.

Made sure that just one file this time, which was the mistake last time.

[github-actions]

🔒 Security Scan Results

🔒 Security Scan Results
=========================

Bandit Scan Results:
-------------------
Run started:2025-09-18 16:00:23.446761

Test results:
	No issues identified.

Code scanned:
	Total lines of code: 1263
	Total lines skipped (#nosec): 0
	Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
	Total issues (by severity):
		Undefined: 0
		Low: 14
		Medium: 0
		High: 0
	Total issues (by confidence):
		Undefined: 0
		Low: 0
		Medium: 0
		High: 14
Files skipped (0):

Dependency Check Results:
-----------------------

✅ No critical security issues detected.

The code has passed all critical security checks.

[Narendran99]

No conflicts. Approving PR.