fix: improve markdown export spacing

[singhvishalkr]

Summary

• add a shared Markdown formatter that separates exported messages with blank lines around ---
• reuse the formatter for full-session and selected-message Markdown exports
• avoid rendering empty raw sources values in selected-message Markdown output

Closes #93

Verification

• python -m pytest backend/tests/test_api.py -k export -q
• python -m pytest backend/tests -q
• python -m py_compile backend/routes/export.py backend/tests/test_api.py
• python -m black --check backend/routes/export.py
• git diff --check

[vercel]

@singhvishalkr is attempting to deploy a commit to the Darshan's projects Team on Vercel.

A member of the Team first needs to authorize it.

[singhvishalkr]

Local verification is green. The Vercel check appears to be blocked on preview authorization for the maintainer team, not on an application build or test failure.

[singhvishalkr]

Updated the branch with current main and resolved the export conflicts.

I also restored the selected-message lookup used by /api/export/messages, which was needed after the merge so the selected Markdown export path stays covered.

Local validation:

• python -m py_compile backend/routes/export.py backend/tests/test_api.py backend/services/db_service.py
• python -m pytest backend/tests/test_api.py -k export -q (9 passed, 39 deselected)
• python -m black --check backend/routes/export.py
• git diff --check origin/main -- backend/routes/export.py backend/tests/test_api.py backend/services/db_service.py

The remaining Vercel status is still preview authorization for the maintainer team.

[singhvishalkr]

Updated the branch with current main and resolved the export conflict in backend/routes/export.py.

Validation:

• python -m py_compile backend/routes/export.py backend/tests/test_api.py backend/services/db_service.py
• python -m pytest backend/tests/test_api.py -k export -q (9 passed, 39 deselected)
• python -m pytest backend/tests/test_api.py -q (48 passed)
• python -m pytest backend/tests/test_csv_parser.py -q (2 passed)
• python -m black --check backend/routes/export.py
• git diff --check

I also tried the full backend test suite, but this local environment is missing optional test dependencies (chromadb, jsonschema) during collection. The focused export path and the newly merged CSV parser tests are passing locally.

The PR is mergeable again. The remaining Vercel status is still the maintainer-team preview authorization check.

[singhvishalkr]

Updated the branch with current main and resolved the selected-message export conflict.

I kept the shared Markdown formatter and the newer safe_title filename behavior from main.

Validation:

• python -m py_compile backend/routes/export.py backend/tests/test_api.py backend/services/db_service.py
• python -m pytest backend/tests/test_api.py -k export -q (9 passed, 39 deselected)
• python -m black --check backend/routes/export.py
• git diff --check origin/main...HEAD

The remaining Vercel status still points to maintainer-team preview authorization.