fix(api): serve React frontend at root instead of redirecting to /docs

The unconditional '/' route redirected to Swagger before the static
mount could serve index.html, so the deployed site showed API docs
instead of the UI. Register the redirect only when no frontend build
exists, and add an SPA fallback so deep links (/runs, /upload, ...)
resolve to index.html on direct navigation.

Author: Goldokpa

src/climatevision/api/main.py

@@ -24,7 +24,7 @@
 from pydantic import field_validator
 
 from fastapi import FastAPI, File, Form, HTTPException, UploadFile, Header, Query, Depends, Request
-from fastapi.responses import RedirectResponse
+from fastapi.responses import FileResponse, RedirectResponse
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.staticfiles import StaticFiles
 from starlette.middleware.base import BaseHTTPMiddleware
@@ -459,10 +459,13 @@ async def lifespan(_app: FastAPI):
 
     # ===== Core Endpoints =====
 
-    @app.get("/")
-    def root() -> RedirectResponse:
-        """Redirect to API docs when no frontend is built."""
-        return RedirectResponse(url="/docs", status_code=302)
+    _frontend_dist = Path(__file__).resolve().parents[3] / "frontend" / "dist"
+
+    if not (_frontend_dist / "index.html").exists():
+        @app.get("/")
+        def root() -> RedirectResponse:
+            """Redirect to API docs when no frontend is built."""
+            return RedirectResponse(url="/docs", status_code=302)
 
     @app.get("/api/health")
     def health() -> dict[str, Any]:
@@ -1198,11 +1201,20 @@ def list_pending_alerts(
 
     # ===== Static Files =====
 
-    # Serve built frontend when dist exists (production mode)
-    frontend_dir = Path(__file__).resolve().parents[3] / "frontend"
-    dist_dir = frontend_dir / "dist"
-    if dist_dir.exists():
-        app.mount("/", StaticFiles(directory=str(dist_dir), html=True), name="frontend")
+    # Serve built frontend when dist exists (production mode).
+    # Registered last so all API routes above take precedence. Unknown paths
+    # fall back to index.html so React Router deep links (/runs, /upload, ...)
+    # work on direct navigation and refresh.
+    dist_dir = _frontend_dist
+    if (dist_dir / "index.html").exists():
+        app.mount("/assets", StaticFiles(directory=str(dist_dir / "assets")), name="assets")
+
+        @app.get("/{full_path:path}", include_in_schema=False)
+        def frontend(full_path: str) -> FileResponse:
+            candidate = dist_dir / full_path
+            if full_path and ".." not in full_path and candidate.is_file():
+                return FileResponse(candidate)
+            return FileResponse(dist_dir / "index.html")
 
     return app