fix: paginate GET /chat (limit + before cursor)

From the PR description

Summary

• Adds limit (default 50, max 200) and before (ISO timestamp cursor) query parameters to GET /chat
• Prevents unbounded chat list queries for active users with large chat histories
• Backward-compatible: existing callers with no query params get the first 50 chats (most recent)

Closes #105 Closes #114

Changes

• backend/src/routes/chat.ts - limit + before-cursor pagination on the chat list query
• backend/src/lib/__tests__/chatPagination.test.ts - static analysis tests verifying pagination is present
• backend/tsconfig.json - exclude __tests__ dirs from the main tsc build (tests use vitest directly)
• backend/package.json - "test": "vitest run" script added

Test plan

• Unit tests added and passing (npx vitest run)
• Backend type check passes (npx tsc --noEmit)