docs: add update procedure section to all READMEs

Explains that db_data/ is preserved across container rebuilds and that
database migrations run automatically on startup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -77,6 +77,19 @@ docker compose --env-file .env up --build -d
 
 ---
 
+## 🔄 Updating
+
+Data is stored in `./db_data/` (bind-mount), which is never touched by container rebuilds. Updating is safe:
+
+```bash
+git pull
+docker compose up --build -d
+```
+
+Database migrations run automatically on startup — no manual steps required.
+
+---
+
 ## ⚙️ Configuration
 
 All configuration is via environment variables. See `.env.example` for the full list with descriptions.