feat: track Flutter version in Renovate via Docker datasource (#452)

## Summary

- Adds a custom Renovate manager that reads the pinned Flutter version from `.fvmrc`
- Uses `ghcr.io/cirruslabs/flutter` as the Docker datasource so Renovate only proposes a bump when the corresponding image tag exists in the registry
- The CI pipeline (`ci/main.go`) already derives the Docker image tag from `.fvmrc` at runtime — `.fvmrc` is the single source of truth; no other files need grouping

## How it works

Renovate checks `ghcr.io/cirruslabs/flutter` for available tags. If `3.44.1` doesn't exist yet, no PR is opened. Once the image is published, Renovate opens a PR to bump `.fvmrc` — the only file that needs to change.

## Verification

- `renovate.json` schema validated
- Reviewed `ci/main.go`: `FlutterVersion` is read exclusively from `.fvmrc`; no hardcoded version strings elsewhere require additional grouping rules

Closes #447

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/452

renovate.json

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