fix(message_builder): add trailing newline to metadata section

The build_metadata_section function used "\n".join(metadata_lines)
which lacks a trailing newline. When dimensions were appended later,
they concatenated directly without proper spacing, producing malformed
output like "**Format:** PNG**Original Dimensions:** 1024x1024".

Added trailing newline so subsequent content appears on a new line.

Fixes issue identified in PR #35 review.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

shared/discord/message_builder.py

@@ -55,7 +55,7 @@ def build_metadata_section(
         return ""
 
     section = f"\n\n**{section_title}:**\n"
-    section += "\n".join(metadata_lines)
+    section += "\n".join(metadata_lines) + "\n"
     return section