Add 4.1.3 plan stub (heavier features)

Reserved for:
1. DCT performance optimizations
2. User management UI (admin CRUD)

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PLAN-4.1.3.md

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+# Stegasoo 4.1.3 Plan
+
+## Release Theme
+Performance and admin features.
+
+---
+
+## 1. DCT Performance Optimizations
+
+**Status:** Planned
+
+**Problem:** DCT encode/decode can be slow on Pi, especially for large images.
+
+**Ideas:**
+- Vectorize block processing with NumPy
+- Reduce Python loop overhead
+- Parallel block processing (multiprocessing?)
+- Profile and identify bottlenecks
+- Consider Cython for hot paths
+
+---
+
+## 2. User Management UI
+
+**Status:** Planned
+
+**Problem:** No way for admin to manage users via UI. Currently need direct DB access.
+
+**Features:**
+- List all users
+- Create new user (admin only)
+- Delete user (admin only)
+- Reset user password
+- User activity/last login
+
+---
+
+## Notes
+
+- These are heavier lifts than 4.1.2
+- Profile before optimizing
+- Consider security implications of user management