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>

composer.json

@@ -0,0 +1,12 @@
+{
+    "name": "unsupervised/freshrss-bluesky-threads",
+    "description": "FreshRSS extension: expand Bluesky posts into full reply threads",
+    "type": "freshrss-extension",
+    "license": "AGPL-3.0-or-later",
+    "require": {
+        "php": ">=8.1"
+    },
+    "require-dev": {
+        "phpunit/phpunit": "^10.5 || ^11.0"
+    }
+}