feat(web): optional web login and API tokens, off by default (stacked on #674) (#683)

Optional web login, off by default: a device that sets no password behaves
exactly as before. Set under General > Security; then every page and API
route needs a session login or an API token (Authorization: Bearer).
Loopback, the Wi-Fi setup flow in AP mode, static files, captive-portal
probes and a reduced /api/v3/health stay open. Secrets live in the web_auth
section of config_secrets.json and no API returns them.
scripts/reset_web_password.py turns login off. Stacked on #674.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/README.md

@@ -37,6 +37,7 @@ display; **diagnostic** — run by hand on a Pi when something is wrong.
 | `prove_security.py` | keep | Security property checks run by pre-commit |
 | `render_bench.py` | diagnostic | Benchmarks the render loop against the panel's real refresh rate on a synthetic strip |
 | `render_plugin.py` | dev-only | Runs a plugin's `update()` + `display()` and saves the frame as a PNG |
+| `reset_web_password.py` | keep | Turns the optional web login off when the password is lost (`sudo python3 scripts/reset_web_password.py`; docs/WEB_INTERFACE_GUIDE.md) |
 | `run_plugin_tests.py` | dev-only | Discovers and runs plugin test suites |
 | `scroll_speeds.py` | keep | Shows and tries the scroll speeds your panel can display cleanly |
 | `troubleshoot_captive_portal.sh` | diagnostic | Troubleshoots captive-portal WiFi setup after you can SSH back in |