Fix video suffix byte range responses

[rissrice2105-agent]

Summary

• parse single video byte ranges in a focused helper
• serve suffix requests such as bytes=-500 from the end of the file
• return 416 Range Not Satisfiable for empty, zero-length, and out-of-bounds ranges
• preserve full responses for absent or unsupported range headers

Tests

• vitest run apps/web/src/range.test.ts --root . (4 passing)
• eslint apps/web/src/actions.ts apps/web/src/range.ts apps/web/src/range.test.ts
• focused TypeScript check for range.ts

Closes #44