Add backend-authoritative invoice draft editing (ADR 0006/0007)

Move the invoice draft editor to a backend single source of truth: an
in-memory InvoiceDraftSession (per-token, cached) holds the editable payload,
server-computed sums/VAT and validation, plus an automatic change history. The
browser posts single edits; the server recomputes and signals the editing
session over a dedicated SignalR hub (DraftPreviewHub) to re-fetch.

This reverses the previously-documented stateless editor (EVAL_live_invoice_editing,
INVOICE_LIFECYCLE §10), by explicit product decision — captured in ADR 0006 and 0007
plus the live-draft-editing concept doc.

Backend (this milestone):
- InvoiceDraftSession + ChangeHistoryEntry data holders
- InvoiceDraftCalculator: pure port of quantChange/invSumUpdate (§13b, VAT-by-rate)
  and consistency checks — fully unit-tested
- IInvoiceDraftCache/InvoiceDraftCache: in-memory store with idle sliding TTL
- IInvoiceDraftService/InvoiceDraftEditService: open (payload or DB reload), patch,
  build state, flush via existing RegisterInvoiceAsync (no new persistence), preview
  from cache, discard (DB reload), history
- InvoiceDraftExpiryService: pre-expiry warning + eviction-with-reason
- DraftPreviewHub + IDraftNotifier/DraftNotifier: targeted draftReady/draftExpiring/
  draftClosed signals per draft token
- inv/dopen|dstate|dpatch|dpreview|dsave|dhistory|ddiscard|dclose endpoints; save
  reports success/failure via the existing EventService
- DI + hub mapping in Program.cs

Frontend (additive foundation): $fis.draft SignalR client for /draftpreview.
The editor DOM inversion (routing deltas, rendering from server state) is the
next, separately-verified step; existing endpoints are unaffected.

Tests: 30 new (calculator, cache, expiry, patch/history, flush); 306 total passing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Fuchs/wwwroot/web/fis.js

@@ -3129,6 +3129,68 @@ $fis.notifications = {
     }
 };
 
+/* Live draft-editing client (ADR 0006/0007). Separate SignalR connection to the
+   dedicated /draftpreview hub; the server signals the *one* browser editing a draft
+   (group = session token) to re-fetch (draftReady), warns before idle expiry
+   (draftExpiring), and tells it to close on eviction (draftClosed). The editor
+   (fis.inv_shared.js) registers the open draft via $fis.draft.bind(token, {...}). */
+$fis.draft = {
+    connection: null,
+    active: null,   /* { token, onReady(version), onExpiring(secondsLeft), onClosed(reason) } */
+    init: function () {
+        if (typeof signalR === 'undefined' || this.connection !== null || !$ocms.auth.useraccount_id) {
+            return;
+        }
+        this.connection = new signalR.HubConnectionBuilder()
+            .withUrl('/draftpreview')
+            .withAutomaticReconnect()
+            .build();
+        this.connection.on('draftReady',    (p) => this._dispatch('onReady',    p, (p) => p.version));
+        this.connection.on('draftExpiring', (p) => this._dispatch('onExpiring', p, (p) => p.secondsLeft));
+        this.connection.on('draftClosed',   (p) => this._dispatch('onClosed',   p, (p) => p.reason));
+        /* Re-join the active draft's group after a (re)connect — group membership is
+           per-connection and is lost when the socket drops. */
+        this.connection.onreconnected(() => { if (this.active) { this._invoke('JoinDraft', this.active.token); } });
+        this.connection.onclose(() => {
+            console.warn('Draft connection closed; retrying in 5s.');
+            this.connection = null;
+            setTimeout(() => { this.init(); if (this.active) { this.bind(this.active.token, this.active); } }, 5000);
+        });
+        this.start();
+    },
+    start: function () {
+        this.connection.start()
+            .then(() => { if (this.active) { this._invoke('JoinDraft', this.active.token); } })
+            .catch((err) => {
+                console.warn('Draft connection failed to start; retrying in 5s.', err);
+                this.connection = null;
+                setTimeout(() => this.init(), 5000);
+            });
+    },
+    /* Registers the currently open draft and joins its signal group. handlers:
+       { onReady, onExpiring, onClosed }. */
+    bind: function (token, handlers) {
+        if (!token) { return; }
+        this.active = $.extend({ token: token }, handlers || {});
+        if (this.connection === null) { this.init(); }
+        this._invoke('JoinDraft', token);
+    },
+    /* Unregisters + leaves the group (editor closed). */
+    release: function (token) {
+        if (this.active && (!token || this.active.token === token)) { this.active = null; }
+        this._invoke('LeaveDraft', token);
+    },
+    _invoke: function (method, token) {
+        if (!token || !this.connection || this.connection.state !== 'Connected') { return; }
+        this.connection.invoke(method, token).catch((err) => console.warn('Draft ' + method + ' failed', err));
+    },
+    _dispatch: function (handler, payload, argOf) {
+        payload = payload || {};
+        if (!this.active || this.active.token !== payload.token) { return; }
+        if (typeof this.active[handler] === 'function') { this.active[handler](argOf(payload)); }
+    }
+};
+
 (function () {
     Array.prototype.push.apply($ocms.ocmsmenu,[
         { lbl: $t.m_inv, id: 'm_inv', fnc: 'init:inv', ico: 'glyphicon glyphicon-list-alt'}
@@ -3143,5 +3205,6 @@ $fis.notifications = {
 })();
 $(document).ready(function () {
     $fis.notifications.init();
+    $fis.draft.init();
     $fis.ov();
 });