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/Docs/Concepts/live-draft-editing.md

@@ -0,0 +1,84 @@
+---
+status: Active
+lastUpdated: 2026-07-10
+applyTo:
+  - "Fuchs/Services/InvoiceDraft*"
+  - "Fuchs/Services/IInvoiceDraft*"
+  - "Fuchs/code/InvoiceDraftSession.cs"
+  - "Fuchs/code/InvoiceDraftCalculator.cs"
+  - "Fuchs/Notifications/DraftPreviewHub.cs"
+  - "Fuchs/Notifications/*DraftNotifier*"
+  - "Fuchs/Controllers/IntranetController.InvoiceDraft.cs"
+  - "Fuchs/js/intranet/**"
+relatedDecisions:
+  - "0006-backend-authoritative-draft-editing.md"
+  - "0007-targeted-draft-signalr-groups.md"
+---
+
+# Live draft editing (backend-authoritative invoice previews)
+
+## Summary
+While a back-office user edits an invoice draft, the authoritative state is held in
+server memory, not in the browser. The browser posts single edits, the server mutates
+the cached record, recomputes totals/VAT and re-validates, then pushes a "state changed"
+signal so the browser re-fetches and re-renders. This makes the backend the single source
+of truth (server-computed sums, consistency checks, in-place PDF preview, change history,
+explicit discard), reversing the earlier stateless editor. Invoices are the pilot;
+reminders are intended to mirror the same design.
+
+## How it works
+
+```
+Open:     Browser --POST inv/dopen {id | payload}--> server builds InvoiceDraftSession, caches it
+          Browser --SignalR JoinDraft(token)--> joins the draft's group; spinner while loading
+          Browser --POST inv/dstate {token}--> renders admin/new/req + server sums + validation
+
+Edit:     Browser --POST inv/dpatch {token, delta}--> mutate + recompute + validate + version++
+          Server  --SignalR draftReady{token,version}--> Browser re-fetches inv/dstate, re-renders
+
+Preview:  Browser --POST inv/dpreview {token}--> PDF rendered straight from the cache (no upload)
+Save:     Browser --POST inv/dsave {token}--> flush cache->DB (RegisterInvoiceAsync) + EventService toast
+History:  Browser --POST inv/dhistory {token}--> change list -> "Änderungshistorie" dialog
+Discard:  Browser --POST inv/ddiscard {token}--> reload session from DB draft -> draftReady
+Close:    Browser --POST inv/dclose {token}--> session removed (+ LeaveDraft)
+
+Expiry:   Server (timer) --SignalR draftExpiring{token,secondsLeft}--> warn "bitte zwischenspeichern"
+          Server (evict) --SignalR draftClosed{token,reason}--> close the editor with a reason
+```
+
+- **Session** (`InvoiceDraftSession`) is a pure data holder: the editable payload as the
+  exact editor JSON (`admin` / `new` / `req` blocks with `items`), plus server-computed
+  `Sums`, `ValidationMessages`, `History`, `Version`, `Token`, `InvId`, `LastAccessUtc`.
+- **Calculation** (`InvoiceDraftCalculator`, static/pure) ports the former client math:
+  `RecomputeItem` (quantity × price × VAT, the `quantChange` port), `RecomputeTotals`
+  (the `invSumUpdate`/`csms` aggregation + §13b reverse-charge), and `Validate`
+  (email/address/items/VAT-rate/negative-total checks). Being pure, it is exhaustively
+  unit-tested.
+- **Orchestration** (`InvoiceDraftEditService`, scoped) opens sessions (from a fresh
+  payload or by reloading a DB draft via `fds__getInvoice`, reshaped like
+  `BuildInvoiceRequestList`), applies deltas (`ApplyDelta`), builds the view-state DTO,
+  flushes to the DB by reusing `IInvoiceService.RegisterInvoiceAsync` (no new persistence
+  path), renders previews from a synthesised registration, and discards by reloading.
+- **Cache** (`InvoiceDraftCache`, singleton) stores sessions by token with an idle sliding
+  TTL; `InvoiceDraftExpiryService` (a `BackgroundService`) warns before, and evicts after,
+  the TTL. TTL/warn-lead are configurable under `Fuchs:DraftEditing`.
+- **Signals** (`DraftPreviewHub` at `/draftpreview` + `IDraftNotifier`) are targeted at the
+  editing browser via a group named after the session token: `draftReady`, `draftExpiring`,
+  `draftClosed`. Business success/failure still flows through `IEventService`/`NotificationHub`.
+- **Frontend** (`$fis.draft` in `fis_main.js`, editor in `fis.inv_shared.js`) opens/joins,
+  posts one delta per change, shows a loading state whenever awaiting a signal, and offers
+  "Änderungen verwerfen" and "Änderungshistorie" menu actions. It no longer computes totals.
+
+## Key files
+- `Fuchs/code/InvoiceDraftSession.cs` — session + `ChangeHistoryEntry` + `InvoiceDraftSums`.
+- `Fuchs/code/InvoiceDraftCalculator.cs` — pure recompute + validate.
+- `Fuchs/Services/InvoiceDraftCache.cs` / `IInvoiceDraftCache.cs` — in-memory store + TTL.
+- `Fuchs/Services/InvoiceDraftEditService.cs` / `IInvoiceDraftService.cs` — orchestration + delta contract.
+- `Fuchs/Services/InvoiceDraftExpiryService.cs` — idle warn/evict monitor.
+- `Fuchs/Notifications/DraftPreviewHub.cs`, `DraftNotifier.cs`, `IDraftNotifier.cs` — targeted signals.
+- `Fuchs/Controllers/IntranetController.InvoiceDraft.cs` — `inv/d*` endpoints.
+- `Fuchs/js/intranet/fis_main.js`, `Fuchs/js/intranet/modules/fis.inv_shared.js` — client.
+
+## Related decisions
+- [0006 — Backend-authoritative draft editing](../Decisions/0006-backend-authoritative-draft-editing.md)
+- [0007 — Targeted draft SignalR groups](../Decisions/0007-targeted-draft-signalr-groups.md)