chore: update .gitignore and add bunfig.toml for test isolation

Exclude runtime data (data/), SQLite WAL files, frontend lock file, and
build artifacts. Add bunfig.toml to scope bun test to src/ only,
preventing it from picking up frontend vitest test files.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

.gitignore

@@ -4,3 +4,17 @@ dist/
 config-overrides.json
 .sisyphus/
 e2e/.env.e2e
+
+# Runtime data
+data/
+
+# Frontend build artifacts
+public/
+
+# Test temporaries
+/tmp/
+*.db-shm
+*.db-wal
+
+# Lock files (frontend has its own)
+frontend/package-lock.json