Show 'Link Discord' only when Discord linking is configured (#213)

The account menu offered 'Link Discord' to every signed-in user even on a
server with no Discord OAuth app configured, where it dead-ends (the start route
303-bounces to /alerts). Add GET /api/v2/discord/config reporting whether linking
is enabled, and have the menu render the entry only when it is. On a server
without Discord set up nothing surfaces; setting the OAuth env vars later makes
the entry appear on its own — no code change needed to turn it on.

discord_link.py

@@ -101,6 +101,15 @@ def _account_redirect(status: str) -> RedirectResponse:
 
 
 # --- routes ------------------------------------------------------------------
+@router.get("/discord/config")
+async def discord_config():
+    """Whether Discord account-linking is configured on this server. The account
+    menu uses it to show the "Link Discord" entry only when linking will actually
+    work — so an unconfigured server surfaces no dead-end Discord UI, and enabling
+    it later (setting the OAuth env vars) makes the entry appear on its own."""
+    return {"enabled": enabled()}
+
+
 @router.get("/discord/link/start")
 async def link_start(request: Request, user=Depends(current_active_user)):
     if not enabled():

tests/test_discord_link.py

@@ -46,6 +46,17 @@ def test_state_is_secret_dependent(monkeypatch):
 
 # --- routes ------------------------------------------------------------------
 
+def test_config_reports_enabled_state(monkeypatch):
+    c = TestClient(appmod.app)
+    monkeypatch.setattr(dl, "CLIENT_ID", "")
+    monkeypatch.setattr(dl, "CLIENT_SECRET", "")
+    r = c.get(f"{V2}/discord/config")   # no auth required
+    assert r.status_code == 200 and r.json() == {"enabled": False}
+    monkeypatch.setattr(dl, "CLIENT_ID", "app123")
+    monkeypatch.setattr(dl, "CLIENT_SECRET", "secret456")
+    assert c.get(f"{V2}/discord/config").json() == {"enabled": True}
+
+
 def test_link_requires_auth():
     c = TestClient(appmod.app)
     assert c.get(f"{V2}/discord/link/start", follow_redirects=False).status_code == 401