build: add Renovate configuration

Track the Dockerfile base image, the actions used in the Gitea workflow,
and versioned front-end dependencies referenced in HTML (via renovate
comment annotations or jsDelivr/unpkg npm URLs).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

renovate.json

@@ -0,0 +1,39 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended",
+    ":dependencyDashboard",
+    ":semanticCommits"
+  ],
+  "labels": ["renovate"],
+  "github-actions": {
+    "fileMatch": ["^\\.gitea/workflows/[^/]+\\.ya?ml$"]
+  },
+  "packageRules": [
+    {
+      "description": "Group nginx base image updates",
+      "matchManagers": ["dockerfile"],
+      "groupName": "docker base image"
+    }
+  ],
+  "customManagers": [
+    {
+      "customType": "regex",
+      "description": "Update HTML dependencies annotated with a renovate comment, e.g. <!-- renovate: datasource=npm depName=bootstrap --> before the versioned URL",
+      "fileMatch": ["\\.html$"],
+      "matchStrings": [
+        "datasource=(?<datasource>\\S+) depName=(?<depName>\\S+)( versioning=(?<versioning>\\S+))?[\\s\\S]*?(?<currentValue>v?\\d+\\.\\d+\\.\\d+[\\w.-]*)"
+      ]
+    },
+    {
+      "customType": "regex",
+      "description": "Auto-detect versioned jsDelivr / unpkg npm assets in HTML",
+      "fileMatch": ["\\.html$"],
+      "matchStrings": [
+        "https://cdn\\.jsdelivr\\.net/npm/(?<depName>@?[^@/]+(?:/[^@/]+)?)@(?<currentValue>\\d[^/\"']+)",
+        "https://unpkg\\.com/(?<depName>@?[^@/]+(?:/[^@/]+)?)@(?<currentValue>\\d[^/\"']+)"
+      ],
+      "datasourceTemplate": "npm"
+    }
+  ]
+}