Add PHPUnit test suite, Gitea CI, and fix broken URL pattern

- Add 22 unit tests covering feed parsing, thread rendering, facets,
  needsRefetch staleness windows, and handleConfigureAction
- Add FreshRSS class stubs so tests run without a full FreshRSS install
- Add RSS feed fixture (hockeyviz.com snapshot) and thread API fixture
  for post 3mhtk7awhrp26 (1 root + 2 replies)
- Add Gitea Actions workflow (.gitea/workflows/ci.yml) running on PHP 8.2
- Fix POST_URL_PATTERN: using '#' as PCRE delimiter with '#' inside a
  character class caused PHP to close the pattern early, so no Bluesky
  URL ever matched; switch delimiter to '~'

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

.gitea/workflows/ci.yml

@@ -0,0 +1,26 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.2'
+          extensions: json, simplexml
+          coverage: none
+
+      - name: Install dependencies
+        run: composer install --no-interaction --prefer-dist
+
+      - name: Run tests
+        run: vendor/bin/phpunit