docs: add superpowers-friendly state and backlog summary

Summarize current v2.4 state and forward-looking backlog in docs/ as a
parallel entry point to the active .planning/ GSD workspace. Lets
superpowers skills (brainstorming → writing-plans → executing-plans)
work from a clean starting point without retiring the existing workflow.

Also ignore .idea/ so JetBrains project files stay local.

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

.gitignore

@@ -233,6 +233,9 @@ e2e/pgdata
 test-results/
 playwright-report/
 
+# JetBrains IDEs (full directory)
+.idea/
+
 # Obsidian
 .obsidian/