feat: syntax highlighting

[darwin67]

Summary

• add the syntax_highlight paste column with a default of plain
• accept and return syntax_highlight metadata through the v1 paste API
• render paste detail pages with Lumis syntax highlighting based on the stored language
• show syntax badges in the paste list and detail views
• add left-side line numbers using Lumis line metadata
• cover the new metadata, highlighting, escaping, and line-number behavior in tests

Verification

• mix test test/textbin_web/live/ui/paste_live_test.exs
• mix precommit