Lehrling, and a field that stops being named after its values

worker_type had two values because the field was named after them:
"Angestellte:r / Arbeiter:in". Lehrlinge are the third social-insurance
category in Austria; until now they were filed as one of the other two,
which they are not -- and which skewed every report grouped by this
column by exactly those people.

Adding the value is one line. The label was the work: the field was
called after its two values in six places, and each of them becomes
wrong with a third. They now read "Beschaeftigtengruppe", the name the
import has used all along.

One label deliberately keeps the old wording: app_feld_karte() in the
database. That string is not a caption there but a key -- stored rows in
employee_history and pending_changes carry it, and the map is how
reverting or correcting a history entry finds the field again. Renaming
it without rewriting those rows would make every older entry for this
field unrevertable, and nobody would notice until they tried.

The migration's assertion reads pg_enum rather than comparing against
'Lehrling'::worker_type: migrations run inside a transaction, and
Postgres refuses to use a freshly added enum value in the transaction
that added it. This has not been run against a live database here -- the
CI migration job is the first real execution.

Three hand-kept lists of the same enum (reports, import, the form) now
have a test holding them to one another, each mutation-checked.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/datenkatalog.md

@@ -302,7 +302,7 @@ eigenen Zeilen sind ohnehin immer dabei. Die Tabelle hiess bis September 2026
 | `employment_status` | Aktiv, Karenz, Geplant, Ausgetreten |
 | `employment_type` | Vollzeit, Teilzeit |
 | `contract_type` | unbefristet, befristet |
-| `worker_type` | Angestellte:r, Arbeiter:in |
+| `worker_type` | Angestellte:r, Arbeiter:in, Lehrling |
 | `collective_agreement` | Handel, Süßwaren |
 | `paygrade_type` | A, B, C, D, E, F |
 | `source_type` | Intern, Extern |