fix: connexion OAuth Gitea — redirect URI invalide (erreur 'Unregistered Redirect URI')

Le /auth/login construisait le redirect_uri avec le schéma http:// en dur
et dupliquait l'expression dans login/callback : toute URL d'accès non
enregistrée (https, reverse proxy, hostname, port différent) était rejetée
par Gitea avec 'Unregistered Redirect URI' — les 2 liens 'Connect Gitea'
et 'Register with Gitea' de Workspaces → Gitea Projects étaient touchés.

- nouveau helper get_redirect_uri(request) : override explicite
  OAUTH_REDIRECT_URI (si défini), sinon schéma depuis X-Forwarded-Proto
  (fallback request scheme) + hôte depuis X-Forwarded-Host (fallback Host)
- redirect_uri stocké en session à l'authorize et réutilisé tel quel dans
  l'échange de code (plus de dérive entre les deux étapes)
- même correctif dans GitHubProvider.exchange_code (ignorait le paramètre)
- config : oauth_redirect_uri par défaut vide (dynamique) au lieu de
  localhost:8080 en dur
- .env.example documente OAUTH_REDIRECT_URI
- 4 tests de régression (host header, X-Forwarded-Proto/Host, override env,
  URL d'authorize) — 178/179 OK, l'échec restant (test_views_calendar) est
  pré-existant et dépend de la date

app/routers/auth.py

@@ -13,6 +13,24 @@ from app.config import settings
 logger = logging.getLogger(__name__)
 router = APIRouter(tags=["auth"], prefix="/auth")
 
+
+def get_redirect_uri(request: Request) -> str:
+    """OAuth redirect URI for this request.
+
+    Explicit `OAUTH_REDIRECT_URI` env override wins (must be registered in the
+    provider's OAuth app). Otherwise it is derived from the request so it always
+    matches the URL the user actually used: scheme from `X-Forwarded-Proto`
+    (reverse proxies) falling back to the request scheme, host from
+    `X-Forwarded-Host` falling back to the `Host` header.
+    """
+    if settings.oauth_redirect_uri:
+        return settings.oauth_redirect_uri
+    proto = request.headers.get("x-forwarded-proto", "")
+    scheme = proto.split(",")[0].strip() or request.url.scheme or "http"
+    fwd_host = request.headers.get("x-forwarded-host", "")
+    host = fwd_host.split(",")[0].strip() or request.headers.get("host", "localhost:8080")
+    return f"{scheme}://{host}/auth/callback"
+
 LOCAL_LOGIN_HTML = """<!DOCTYPE html>
 <html lang="en">
 <head>
@@ -144,10 +162,11 @@ async def login(request: Request, provider: str = Query("gitea")):
     # Encode auth mode in state to survive session loss during OAuth redirect
     signed_state = f"{state}:{mode}" if mode else state
     request.session["oauth_mode"] = mode
-    # Dynamic redirect URI based on incoming Host header
-    host = request.headers.get("host", "localhost:8080")
-    dynamic_redirect_uri = f"http://{host}/auth/callback"
-    auth_url = oauth_provider.get_authorize_url(signed_state, redirect_uri=dynamic_redirect_uri, force_login=(mode == "link"))
+    # Redirect URI derived from the incoming request (scheme-aware); stored in
+    # session so the callback reuses the EXACT same URI for token exchange
+    redirect_uri = get_redirect_uri(request)
+    request.session["oauth_redirect_uri"] = redirect_uri
+    auth_url = oauth_provider.get_authorize_url(signed_state, redirect_uri=redirect_uri, force_login=(mode == "link"))
     return RedirectResponse(url=auth_url, status_code=302)
 
 
@@ -282,10 +301,10 @@ async def callback(
     if not oauth_provider:
         return HTMLResponse(f"<h1>Unknown provider: {provider_name}</h1>", status_code=400)
 
-    # Exchange code for token — use dynamic redirect URI matching the authorize step
-    host = request.headers.get("host", "localhost:8080")
-    dynamic_redirect_uri = f"http://{host}/auth/callback"
-    token_data = await oauth_provider.exchange_code(code, redirect_uri=dynamic_redirect_uri)
+    # Exchange code for token — reuse the redirect URI from the authorize step
+    # (stored in session), falling back to deriving it from this request
+    redirect_uri = request.session.get("oauth_redirect_uri") or get_redirect_uri(request)
+    token_data = await oauth_provider.exchange_code(code, redirect_uri=redirect_uri)
     if not token_data:
         return HTMLResponse("<h1>Token exchange failed</h1>", status_code=400)