Fix Python scoping issue: rename html variable to avoid conflict

The variable 'html' was being assigned to store HTML content, which
caused Python to treat 'html' as a local variable throughout the
function. This prevented access to the 'html' module (imported at
the top) within f-strings that referenced html.escape().

Renamed the HTML content variable to 'html_content' to avoid the
naming conflict with the html module.

internal/assets/migrations/000005_oidc_keys.up.sql

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS "oidc_keys" (
+    "id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+    "private_key" TEXT NOT NULL,
+    "created_at" INTEGER NOT NULL,
+    "updated_at" INTEGER NOT NULL
+);
+