chore(ci): .dockerignore + path filters on existing workflows

The repo now contains the Flutter mobile client at flutter_client/
(M7 #356). Two cleanups so the existing build pipelines don't churn
on Flutter-only changes:

1. .dockerignore — keep the Flutter project (and docs/, IDE noise,
   git metadata) out of the Docker build context. The runtime image
   was already unaffected (multi-stage drops everything but the Go
   binary), but the COPY . . layer cache invalidated on every
   Flutter edit. Build context shrinks ~2.4 MB and Flutter-only
   pushes no longer force Go rebuilds.
2. test.yml + release.yml — paths-ignore on flutter_client/**,
   docs/**, and **/*.md. Go/web tests + container build now skip
   when the diff doesn't touch them. Tag releases still build broadly
   since server + Flutter ship paired.

The Flutter-side workflow lands in M7 Task 21 with its own paths
filter restricting it to flutter_client/** and the shared web/
inputs (tokens.json, error-copy.json, album-fallback.svg).

.forgejo/workflows/release.yml

@@ -10,6 +10,14 @@ on:
   push:
     branches: [main]
     tags: ['v*']
+    # Tag pushes still build (tag releases are intentional, server + Flutter
+    # share the version). Branch pushes skip when the diff is Flutter-only —
+    # rebuilding the Go image for a Flutter-only merge wastes CI and churns
+    # the registry.
+    paths-ignore:
+      - 'flutter_client/**'
+      - 'docs/**'
+      - '**/*.md'
   workflow_dispatch:
 
 jobs: