chore(release): bump version to 0.5.15

[SushantGautam]

Release 0.5.15

New features

• Langfuse chat tracing — OWUI filter function captures full input/output/model/tokens/metadata per chat completion, auto-binds on every server restart
• OpenAI-compatible API — expose agents/models via OpenAI-compatible endpoint with personal API key
• Versioned system prompts — diff view and per-version notes for prompt management
• SEO share cards — Open Graph metadata for public experiment pages, gallery, and login

Fixes

• OWUI v0.11 filter API compatibility (inlet/outlet handlers, filterIds binding)
• Tracing function and stub aligned on top-level chat_id
• Dev server port handling

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com