[P3] Test coverage gaps: junitParser, scanJobs, templates, dataMigration, video

[eboudrant]

Priority: P3 · Review finding: #23

Modules with no direct unit tests (all fit the existing node:test + tmpdir pattern):

• src/junitParser.js — stats aggregation, diff-percentage extraction, malformed XML (only indirectly covered via scanner tests)
• src/scanJobs.js — job lifecycle, cancellation, needs_confirmation parking/TTL, "tarball always cleaned up on terminal transition" invariant
• src/templates.js — CRUD, id collision with built-ins ([P1] Custom template id can collide with a built-in → shadowed, undeletable template #133)
• src/dataMigration.js — v1→v2 migration, backup creation, idempotency
• src/video.js — pure parts (PNG encoder, crc32, overlay rendering dimensions)

Plus regression tests once the related bugs are fixed: size=0 passthrough (#131), delete-project scan cascade (#128), Accept-All-skips-rejected (#129).