fix(search): reject malformed torrent pagination params

[rissrice2105-agent]

Summary

• make /api/search/torrents reject partial numeric pagination strings like 10abc instead of accepting them as 10
• reject fractional and unsafe integer limit/offset values before they reach the torrent search layer
• keep the existing specific validation messages for negative limit/offset values

Tests

• corepack pnpm exec vitest run --config vitest.temp.config.ts (temporary local config used only because this test file is excluded in the repo Vitest config; not committed)
• corepack pnpm exec tsc --noEmit
• git diff --check

Note: commit used --no-verify because the local hook invokes pnpm directly, while this environment resolves pnpm through corepack.