feat: self-serve signup Phase 2 (frontend cutover) (#162)

Co-authored-by: Michael Chihlas <michael@resolutionflow.com>
Co-committed-by: Michael Chihlas <michael@resolutionflow.com>

backend/app/api/endpoints/config.py

@@ -0,0 +1,40 @@
+"""Public runtime configuration endpoint.
+
+GET /api/v1/config/public
+  Returns the small set of runtime flags the frontend needs at app load
+  to decide whether to render the self-serve signup flow and which OAuth
+  buttons to show. No authentication required.
+
+The response model lives in `app.schemas.config` so it can be reused by
+frontend codegen and other call sites if needed.
+"""
+
+from __future__ import annotations
+
+from fastapi import APIRouter
+
+from app.core.config import settings
+from app.schemas.config import PublicConfigResponse
+
+router = APIRouter(prefix="/config", tags=["config"])
+
+
+@router.get("/public", response_model=PublicConfigResponse)
+async def get_public_config() -> PublicConfigResponse:
+    """Return public-safe runtime config.
+
+    `oauth_providers` reflects which OAuth client IDs are configured server
+    side; the frontend uses it to render only buttons that will actually
+    succeed. `self_serve_enabled` is the master switch for the new public
+    self-serve signup flow.
+    """
+    providers: list[str] = []
+    if settings.GOOGLE_CLIENT_ID:
+        providers.append("google")
+    if settings.MS_CLIENT_ID:
+        providers.append("microsoft")
+
+    return PublicConfigResponse(
+        self_serve_enabled=settings.SELF_SERVE_ENABLED,
+        oauth_providers=providers,
+    )