Add dual user classes: admin + tech users with PIN login

- Add tech user management (JSON-backed CRUD with PIN auth)
- Dual login: tabbed Tech Login (username+PIN) / Admin Login (NC credentials)
- Admin panel: tappable user list with detail modal (enable/disable, reset PIN, reset NC password, delete)
- Auto-provision Nextcloud accounts for tech users
- Admin guard: tech users redirected away from admin panel
- New data volume for persistent tech_users.json storage

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

Dockerfile

@@ -18,7 +18,7 @@ RUN pip install --no-cache-dir -r requirements.txt
 COPY . .
 
 # Create directories for runtime data
-RUN mkdir -p /tmp/flask_session && \
+RUN mkdir -p /tmp/flask_session /app/data && \
     chmod 777 /tmp/flask_session
 
 # Create non-root user for security