Document WebUntis

docs/API.md gains the findings: the endpoint and its required version
parameter, the one-time code and the two error codes worth naming, the Z that
means local time, a substitution being two periods, the unused exam module
that puts announced tests in the period notes, and a day without lessons that
is not a holiday. Those cost an afternoon of probing to learn and nothing
upstream states them.

docs/AUTH.md sets the key against the Schulcloud token it sits beside: no
password, nothing to keep alive, revocable on its own, and not read-only in
itself — which is why the allowlist exists. PI.md and DEPLOYMENT.md add the
four values, the container recreate a changed key needs, and the clock
requirement; the Pi's troubleshooting table gains both failure messages.

README and CLAUDE.md say what the server now is: Schulcloud for the material,
WebUntis for the day. Smoke is 89 checks with the index and a key, 87
live-only, 9 fewer without one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/LOCAL.md

@@ -107,9 +107,9 @@ node dist/bin/cli.js sync
 ## Run the test suites
 
 ```bash
-npm test              # 184 offline tests
+npm test              # 224 offline tests
 npm run smoke         # end-to-end against the live instance, live-only mode
-DATABASE_URL=… npm run smoke    # end-to-end with the index (79 checks)
+DATABASE_URL=… npm run smoke    # end-to-end with the index (89 checks with a WebUntis key)
 ```
 
 Store tests need a database and skip without one: