feat(backend): Taiga-Proxy — vier schmale Endpunkte, Ziel-URL aus der Server-Konfiguration (D91, #trk.create.proxy)

API First: /taiga/auth, /taiga/projects, /taiga/userstories, /taiga/tasks
in der OpenAPI-Spec; TaigaClient/TaigaProperties in
de.werkbaum.integration.taiga. Die API-URL kommt aus
WERKBAUM_TAIGA_API_URL (nie Request-Parameter — SSRF), das Token je
Aufruf im Header X-Taiga-Token (Authorization muessen OpenAPI-Werkzeuge
als Header-Parameter ignorieren) und geht als Bearer hinaus; der Server
speichert nichts und loggt keine Request-Bodies. Taiga-4xx werden samt
_error_message durchgereicht, 5xx/Netz sind 502, unkonfiguriert 503 —
und GET /info meldet das Feature (taiga). Tests gegen aufgezeichnete
Antwortformen auf einem JDK-HttpServer-Stub (statt WireMock: keine neue
Test-Abhaengigkeit, dieselbe Zusicherung); Gegenprobe: ohne den
type-Durchreich faellt genau der benannte Test. check gruen, 93 %
Coverage.

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

docs/CHANGELOG.md

@@ -19,6 +19,7 @@ reverse.
 
 ## 2026-08-27
 
+- The backend proxies Taiga through four named endpoints — login, projects, create story, create task; the Taiga API address is server configuration, the token stays in the browser, and `/api/v1/info` says whether the feature is on
 - Free `&tag` keywords are part of the notation now: free-standing only, quoted mentions like `(&taiga.slug)` stay labels — and `&taiga.slug` names the Taiga project of a subtree, inherited downwards
 - Plans spanning several Taiga projects: a `&taiga.slug` tag names the project per subtree, inherited downwards — the first consumer of the reserved free tags, recorded as planned
 - Creating Taiga tickets from nodes recorded as planned: a story per node from the node window, sub-packages as tasks picked in a dialog, through a backend proxy — the ref lands in the line as `#US-123`/`#T-1234`, beside the node id

docs/examples/werkbaum.werkbaum

@@ -206,7 +206,7 @@
         - [ ] #trk.resolve.map: Map the workflow onto the states (S)
       - [?] #trk.write: Write the status back (M) :#trk.resolve
       - [ ] #trk.create: Create tickets from nodes (XL)
-        - [ ] #trk.create.proxy: Backend proxy with named endpoints (M)
+        - [x] #trk.create.proxy: Backend proxy with named endpoints (M)
         - [ ] #trk.create.login: Log in to Taiga, token stays in the browser (S) :#trk.create.proxy
         - [ ] #trk.create.project: The project comes from the inherited tag (S) :#not.tag.project
         - [ ] #trk.create.story: A "create story" action in the node window (S) :#trk.create.login