chore: untrack Garmin token store, ignore token/db-sidecar/IDE files

backend/.garmin (live Garmin OAuth tokens) was accidentally swept into
1ba7724 by a broad git add; stop tracking it and ignore it, along with
SQLite -shm/-wal sidecars and the rest of .idea.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -24,3 +24,11 @@ backend/cmd/mcpspike/mcpspike
 
 # Claude Code session-local runtime state
 .claude/*.lock
+
+# Garmin token stores hold live OAuth tokens -- never track them.
+backend/.garmin/
+# SQLite sidecar files
+*.db-shm
+*.db-wal
+# IDE state beyond the shared bits already ignored above
+.idea/