Fix lint and docker-build: sprintf→snprintf + add libmicrohttpd

[AlanRuno]

Summary

This PR fixes two CI issues:

1. Lint fix (sprintf → snprintf):
Replaces unsafe sprintf calls with snprintf to satisfy the CI lint check that flags printf-family functions as potentially unsafe. This is a mechanical change with no functional impact.

Files changed:

• src/mxd_http_api.c - 5 sprintf → snprintf replacements for block hash and address hex conversion
• src/blockchain/mxd_rsc.c - 4 sprintf → snprintf replacements for genesis address logging

All buffer sizes are verified correct:

• 129 bytes for 64-byte hashes (128 hex chars + null terminator)
• 41 bytes for 20-byte addresses (40 hex chars + null terminator)
• snprintf size of 3 per iteration (2 hex chars + null, which gets overwritten by next iteration)

2. Docker build fix:
Adds missing libmicrohttpd dependency that was causing docker-build CI to fail with "No package 'libmicrohttpd' found".

Dockerfile changes:

• Builder stage: added libmicrohttpd-dev
• Runtime stage: added libmicrohttpd12 and curl (curl needed for existing healthcheck)

Review & Testing Checklist for Human

• Verify the snprintf size parameter 3 is correct for the "%02x" format (writes 2 chars + null per byte)
• Verify docker-build CI passes with the new dependencies
• Test Docker image builds and runs correctly (healthcheck uses curl)

Test plan:

1. Verify gcc lint check passes in CI
2. Verify docker-build job passes in CI
3. Optionally: docker build . locally and verify the image starts

Notes

Link to Devin run: https://app.devin.ai/sessions/f0a6357d898e440ea3745d77dfb91d8d
Requested by: Runo (runonetworks@gmail.com) / @AlanRuno

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