Remove hardcoded /api/v2/ prefixes from backend URLs (follow-up to #6938)

[suzit-10]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Describe this PR

This PR cleans up redundant /api/v2/ prefixes across API endpoints, following up on #6938
contributed by @jake-low.

Background
In #6938, @jake-low removed hardcoded /api/v2 prefixes to support non-standard backend URLs. While that PR fixed the main issue, there were additional occurrences of /api/v2/ prefixes across the codebase. This PR rebases and extends their work, applying the cleanup consistently.

Technical Note
The URL constructor merges relative paths with the base URL automatically:

const url = new URL('/api/v1/', 'https://tasks.hotosm.org');
const newUrl = new URL('/api/v1/projects/', url);

console.log(newUrl.href);
// https://tasks.hotosm.org/api/v1/projects/

Even when both the base and relative path include /api/v1/, the final URL is correct.
This ensures that removing the redundant prefixes is safe, and no changes to fetchLocalJSONAPI() were required.

Changes

• Removed all redundant /api/v2/ prefixes across the codebase.
• Verified all API calls continue to work as expected.

Outcome

• Cleaner, consistent, and maintainable API paths
• No functional changes or broken endpoints
• Finalizes and extends Make auth callback URL relative to base API_URL #6938
• After merging, the original PR Make auth callback URL relative to base API_URL #6938 can be closed

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud