fix(api): validate module pagination

[lazyGPT07]

Summary

• validate module page and limit query parameters as positive safe integers
• fall back to documented defaults for malformed, fractional, zero, negative, and non-finite values
• preserve the 50-item page-size cap
• add focused pagination regressions

Validation

• modules API suite: 18 tests passed
• web TypeScript check passed after clearing stale generated .next types
• git diff --check passed

Closes #10