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>

phpunit.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="tests/bootstrap.php"
+         colors="true">
+    <testsuites>
+        <testsuite name="BlueskyThreads">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+</phpunit>