ci: replace upstream's GHCR release workflow with a Gitea CI (tests + image build)

Upstream's build.yaml pushes to ghcr.io and has failed on every push since the
fork — those reds were never ours. Gitea now runs the 17 tests and a docker
build with no push.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014LCQmFvKshyAJsE8C3B5ET

.gitea/workflows/ci.yml

@@ -0,0 +1,19 @@
+# Gitea CI for the OTM fork: the unit + Flask tests, then prove the image builds.
+# Upstream's .github/workflows/build.yaml pushed to GHCR and could never pass here.
+name: CI
+on:
+  push:
+  pull_request:
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    container: python:3.13-slim
+    steps:
+      - uses: actions/checkout@v4
+      - run: pip install -q -r requirements.txt
+      - run: cd app && python -m unittest -v
+  image:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: docker build -t 13ft-ci:${{ gitea.sha }} .