fix(web): reject empty values in PUT /api/env

The endpoint accepted empty strings, allowing any .env key to be
silently blanked out from the web UI. Add Pydantic validators to
reject empty keys and values.

hermes_cli/web_server.py

@@ -53,7 +53,7 @@ try:
     from fastapi.middleware.cors import CORSMiddleware
     from fastapi.responses import FileResponse, HTMLResponse, JSONResponse
     from fastapi.staticfiles import StaticFiles
-    from pydantic import BaseModel
+    from pydantic import BaseModel, field_validator
 except ImportError:
     raise SystemExit(
         "Web UI requires fastapi and uvicorn.\n"
@@ -425,6 +425,20 @@ class EnvVarUpdate(BaseModel):
     key: str
     value: str
 
+    @field_validator("key")
+    @classmethod
+    def key_must_be_nonempty(cls, v: str) -> str:
+        if not v.strip():
+            raise ValueError("key must not be empty")
+        return v
+
+    @field_validator("value")
+    @classmethod
+    def value_must_be_nonempty(cls, v: str) -> str:
+        if not v.strip():
+            raise ValueError("value must not be empty; use DELETE /api/env to remove a key")
+        return v
+
 
 class EnvVarDelete(BaseModel):
     key: str