Expense search leaks other groups' expenses #4

What happened

The expense search endpoint (GET /api/groups/:groupId/expenses?q=...)
builds its SQL query by splicing the q parameter directly into a LIKE
clause instead of binding it as a parameter. A crafted q breaks out of
the clause and can read expenses that belong to a completely different
group than the one in the URL and the caller's token.

Steps to reproduce

Using a Goa trip member's token, search Goa trip's own expenses with a
crafted query:

curl -s "http://localhost:3200/api/groups/goa-trip/expenses?q=%27%20OR%201%3D1%20--" \
  -H "Authorization: Bearer goa-asha"

(q is ' OR 1=1 --.)

Expected

Only Goa trip's own expenses (matching the search term, or none) come
back.

Actual

The response includes Flat 4B's expenses too - a group goa-asha is not a
member of and has no token for.

Area

search

Severity

high

Notes for whoever picks this up

See .claude/skills/fixing-a-vulnerability: write the exploit above as a
failing test first (assert the response does NOT contain a Flat 4B
expense), then fix src/expenses.ts's listExpenses to bind query as a
? parameter instead of splicing it into the SQL string.