Normalise Events type filters

[d180]

Summary

Normalise the Events type filter before applying the store query so llm and tool map to the stored EVENT_TYPE_LLM and EVENT_TYPE_TOOL values.

This fixes the dashboard behaviour where selecting LLM or Tool could hide matching events because the frontend filter values did not match the exact values stored in
events.event_type.

Closes #9

Test plan

• docker run --rm -v "$PWD":/src -w /src/backend golang:1.25-bookworm go test ./internal/store

Checklist

• CLA signed (see CLA.md)
• Tests pass locally
• Docs updated where needed
• British English; no em-dashes; no marketing fluff