Add Cloudinary storage provider

[saurabhhhcodes]

Summary

• add a Cloudinary-backed storage provider for resume PDF upload, download, and delete flows
• wire STORAGE_PROVIDER=cloudinary through backend settings and provider selection
• document Cloudinary environment variables and add mocked provider tests
• fix backend test metadata setup for DSA questions so the SQLite test suite can create all tables

Closes #32

Validation

• python3 -m uv run pytest tests/test_utils/test_cloudinary_provider.py -q 6 passed
• python3 -m uv run pytest -q 57 passed
• python3 -m uv run ruff check . All checks passed

Notes

• End-to-end Cloudinary upload/download testing requires valid CLOUDINARY_CLOUD_NAME, CLOUDINARY_API_KEY, and CLOUDINARY_API_SECRET credentials. Unit tests mock Cloudinary and network calls.

GSSoC / NSoC label request

Please add scoring labels if this is accepted. Current repo labels only expose gssoc, gssoc'26, gssoc2026, and gssoc:approved; please apply the appropriate GSSoC/NSoC scoring labels such as approval, level, quality, and backend/feature type labels if available.

[saurabhhhcodes]

Fixed the CI format failure from ruff format --check . and pushed a formatting-only follow-up commit.

Validation after the fix:

• python3 -m uv run ruff format --check . — passed
• python3 -m uv run ruff check . — passed
• python3 -m uv run pytest tests/test_utils/test_cloudinary_provider.py -q — 6 passed
• git diff --check — passed

The previous red job was only formatting drift; no behavior changes were needed.

[saurabhhhcodes]

Pushed one more CI repair for the mypy failure:

• narrowed the Cloudinary upload URL response to a non-empty str before returning it, fixing no-any-return

Validation now passing locally:

• python3 -m uv run ruff format --check .
• python3 -m uv run ruff check .
• python3 -m uv run mypy .
• python3 -m uv run pytest tests/test_utils/test_cloudinary_provider.py -q — 6 passed
• git diff --check

Watching the fresh Backend CI rerun now.

[saurabhhhcodes]

Fresh Backend CI is green now on commit 6c50736.

GitHub check passed:

• Lint · Format · Type-check (Ruff + Mypy) — success

The two earlier failed Backend CI notifications were fixed by the formatting and mypy follow-up commits. PR is mergeable again and ready for review. Could you please add the relevant GSSoC labels when reviewing, such as gssoc:approved, difficulty, type:feature, and backend/storage labels? Thanks!

[saurabhhhcodes]

Gentle label follow-up: this PR is linked to #32, the repository check Lint · Format · Type-check (Ruff + Mypy) is green, and merge state is clean. Could you please add the appropriate GSSoC/scoring labels during review, such as gssoc:approved, a difficulty label, and type:feature/type:backend if they match the project convention?