fix(cors): allow Tauri desktop origins so v0.4.0 agent fetches don't fail

User report after the v0.4.0 desktop agent landed:
> agente devuelve "LLM call failed: Failed to fetch"

"Failed to fetch" is a network-layer error, not 401. Root cause: the
OSS CORS allow_origins list only included http://localhost:517[3-5]
(vite dev) and settings.FRONTEND_URL. The desktop bundle runs from
either tauri://localhost (macOS/Linux) or http://tauri.localhost
(Windows) - both cross-origin to velxio.dev - so the browser
blocked the agent's POST /api/pro/agent/llm preflight before the
backend ever saw it.

Added all three Tauri scheme variants to the allow list. After this
lands + a backend restart the desktop agent's fetch reaches the
real /api/pro/agent/llm and the dual-auth dep from v0.4.0 Phase 1
gets to do its job (Bearer license-key → resolved User → quota
check → upstream LLM proxy).

Origins added:
  tauri://localhost          # macOS / Linux (Tauri 2.x default)
  http://tauri.localhost     # Windows (Tauri 2.x default)
  https://tauri.localhost    # older Tauri 2.x releases

allow_credentials stays True - the existing cookies-from-web flow
still works, the Tauri origins just don't have any cookies to send.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -62,13 +62,26 @@ app = FastAPI(
     openapi_url="/api/openapi.json",
 )
 
-# CORS for local development
+# CORS — local Vite dev, the prod web origin, AND the Velxio Desktop
+# Tauri origins. The desktop bundle runs from a non-http scheme so
+# every fetch to velxio.dev is cross-origin and the browser blocks
+# preflight unless we explicitly allow the Tauri scheme(s).
+#
+# Tauri origin per OS:
+#   - macOS / Linux: `tauri://localhost`
+#   - Windows:       `http://tauri.localhost`
+#   - older Tauri:   `https://tauri.localhost`
+# All three are listed so the desktop bundle works regardless of
+# host OS or Tauri version.
 app.add_middleware(
     CORSMiddleware,
     allow_origins=[
         "http://localhost:5173",
         "http://localhost:5174",
         "http://localhost:5175",
+        "tauri://localhost",
+        "http://tauri.localhost",
+        "https://tauri.localhost",
         settings.FRONTEND_URL,
     ],
     allow_credentials=True,