fix: normalize all repeated path slashes

[vagdotdev]

Summary

• collapse every repeated slash run during path normalization, not just the first one
• keep download query lookups consistent for paths containing multiple duplicate slash groups
• add regression coverage for /files/download?file_path=...
• make test servers bind to OS-assigned ports so parallel CI runs do not collide

Validation

• bun test tests/routes/files.test.ts
• bun test && bun test && bun test
• bunx tsc --noEmit
• bun run format:check
• bun run build
• git diff --check

/claim #5

[vagdotdev]

/claim #5