docs(readme): harden jpg_stream_httpd_handler example & fix bugs

[RubenKelevra]

Description

Changes:

• Use %zu in _STREAM_PART for size_t-safe Content-Length formatting.
• Fix part_buf type: char part_buf[64] (was array of char pointers).
• initialize local JPEG buffer vars: jpg_buf_len/jpg_buf
  • drop leading underscores on them for consistency with other locals
• Bound snprintf() by sizeof(part_buf); capture int hlen.
• Detect and log header truncation with required/available sizes; fail gracefully.
• Break out of loop on JPEG compression failure to avoid invalid buffer use.
• Send header chunk only when formatting succeeds.
• Guard FPS calculation against divide-by-zero; compute once as fps.
• Remove unsafe/needless pointer cast.
• Minor style cleanups for readability.

Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.