chore(gitignore): ignore precedent-library data, .db files, .bak backups

After committing the Paperclip gaps refactor, the .bak-pre-* sentinels
served their purpose. Add a wildcard so future similar backups won't be
tracked. Also ignore data/precedent-library/ (binary PDFs, 11MB) and
data/*.db (sqlite caches).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -3,7 +3,10 @@ data/cases/
 data/training/
 data/exports/
 data/backups/
+data/precedent-library/
 data/.auto-sync.log
+data/*.db
+*.bak-pre-*
 mcp-server/.venv/
 __pycache__/
 *.pyc