feat: track Flutter version in Renovate via Docker datasource

Adds a custom Renovate manager that reads the pinned Flutter version
from .fvmrc and validates new versions against the
ghcr.io/cirruslabs/flutter Docker registry. Renovate will only open a
bump PR when the corresponding image tag actually exists, preventing
CI breakage from unavailable images.

Closes #447

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

renovate.json

@@ -19,6 +19,14 @@
     }
   ],
   "customManagers": [
+    {
+      "customType": "regex",
+      "fileMatch": ["^\\.fvmrc$"],
+      "matchStrings": ["\"flutter\":\\s*\"(?<currentValue>[^\"]+)\""],
+      "depNameTemplate": "ghcr.io/cirruslabs/flutter",
+      "datasourceTemplate": "docker",
+      "versioningTemplate": "semver"
+    },
     {
       "customType": "regex",
       "fileMatch": ["^\\.forgejo/Dockerfile$"],