Show positions that do not exist yet, and let them be corrected

Two gaps in the positions view, both reported from use.

A position dated into the future was invisible. loadOpenPositions required
valid_from <= today, so a position decided now and effective at the quarter
boundary appeared nowhere until the day it began. The database already held
one — 60000824 "Neue Position", effective 01.09. — created through the
application and shown on no screen since.

Future positions now have their own section rather than joining the vacancy
list. They are a different statement: "nobody is here" and "this does not
exist yet" should not be counted together, and a position starting 01.10.
read as a vacancy nobody was filling.

Positions could only be created and deleted. Fixing a typo in the job title
meant deleting and recreating — with a new position number, which appears in
job postings, budgets and audit entries, and whose trail then breaks.
update_position keeps the number and records old and new values per field,
using the audit detail added earlier today.

Three things it refuses, as guards rather than remarks:

  - Moving an occupied position to another unit. That is a transfer, with
    history and reporting line, and belongs to the person — otherwise
    someone changes department silently.
  - Ending an occupied position, which would leave an assignment without
    one.
  - A second chief position in a unit, or an end before the start.

Verified against the live database, all rolled back: each guard fires with
its own message, the permitted edits go through, the audit entry carries the
changed fields. Open positions stay at 9 and the future one now appears in
its own section.

ESLint caught me priming the dialog's fields from an effect. Replaced by a
key on the component, so React rebuilds it per position and the fields
initialise from props — which also removes the flash of the previous
position's values on second open.

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

lib/supabase/types.ts

@@ -457,6 +457,7 @@ export type Database = {
       // Planstelle anlegen bzw. schliessen — im OM-Modell Operationen auf
       // om_positions, nicht mehr auf einer eigenen Ausschreibungstabelle.
       create_position: { Args: { payload: Record<string, unknown> }; Returns: string };
+      update_position: { Args: { payload: Record<string, unknown> }; Returns: void };
       delete_position: { Args: { payload: Record<string, unknown> }; Returns: void };
       is_valid_svnr: { Args: { p_svnr: string; p_birth_date?: string | null }; Returns: boolean };
       apply_due_pending_changes: { Args: Record<string, never>; Returns: number };