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/test.yml

@@ -8,8 +8,15 @@ on:
   # CI on their own — opening a PR fires the pull_request event and gates
   # the merge. Avoids the duplicate push+pull_request runs we were
   # accumulating on every PR commit.
+  #
+  # paths-ignore skips this workflow when only Flutter-client files change;
+  # the Flutter workflow (.forgejo/workflows/flutter.yml) handles those.
   pull_request:
     branches: [main]
+    paths-ignore:
+      - 'flutter_client/**'
+      - 'docs/**'
+      - '**/*.md'
 
 jobs:
   test: