refactor: merge setup.sh into update.sh for seamless first-run

- update.sh now auto-generates .env from .env.example if missing
- Generates random JWT/SALT keys on first run
- Existing .env is never overwritten
- Deleted redundant setup.sh
- Updated all documentation references

docs/superpowers/specs/2026-06-21-nexusguard-portfolio-design.md

@@ -172,7 +172,7 @@ Technical and precise. Avoid marketing buzzwords. Focus on:
 
 **Content:**
 - Docker (Recommended):
-  - Quick start (setup.sh + update.sh)
+  - Quick start (bash update.sh)
   - Configuration (.env)
   - Makefile commands (up, down, logs, dev, migrate, reset-db)
 - Native Install: