feat: add admin preview endpoint with tests

[shravanithouta108]

Description

Implements a POST /admin/preview/ endpoint that allows admins to preview how prompt templates render with placeholder variables before running full analyses. Optionally returns a mock provider response.

Related Issue

Fixes #651

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Test evidence

tests/test_preview.py::test_preview_basic PASSED
tests/test_preview.py::test_preview_missing_variable PASSED
tests/test_preview.py::test_preview_with_mock_response PASSED
tests/test_preview.py::test_preview_unauthorized PASSED
4 passed

##Demo

Demo

Tested via Swagger UI at POST /admin/preview/:

• ✅ Template renders correctly with variables
• ✅ Mock provider response returned
• ✅ Variables found/missing tracked correctly
• ✅ Unauthorized access blocked (403)

demo.mp4

[shravanithouta108]

Hi @imDarshanGK! 👋

Just following up on this PR — it's been a few days and I wanted to check if you've had a chance to look at it.

All 4 tests are passing and the branch can be cleanly merged. Happy to make any changes based on your feedback!

Thank you for your time. 🙏

[imDarshanGK]

@shravanithouta108 add demo

[shravanithouta108]

Hi @imDarshanGK! 👋

Added the demo video showing the POST /admin/preview/ endpoint working via Swagger UI.

Also fixed the lint issues:

• Removed unused Depends import from preview.py
• Removed trailing whitespace in main.py

All 4 tests still passing. Ready for review!

[imDarshanGK]

@shravanithouta108

[shravanithouta108]

Hi @imDarshanGK! 👋

Following up on PR #882 — it's been a few days since your last comment. I've applied black formatting as requested and all 4 tests are still passing ✅.

The only remaining issue is the merge conflict in backend/app/main.py and backend/app/routers/analyze.py — I'll resolve those right away and push an update.

Could you please take another look once that's done? Happy to make any other changes you need. 🙏

[imDarshanGK]

@shravanithouta108 resolve conflicts

[shravanithouta108]

Resolved merge conflicts in main.py and analyze.py. Ready for review!