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>

extension.php

@@ -5,7 +5,7 @@ declare(strict_types=1);
 final class BlueskyThreadsExtension extends Minz_Extension {
 
 	private const API_BASE = 'https://public.api.bsky.app/xrpc';
-	private const POST_URL_PATTERN = '#bsky\.app/profile/([^/]+)/post/([^/?#\s]+)#';
+	private const POST_URL_PATTERN = '~bsky\.app/profile/([^/]+)/post/([^/?#\s]+)~';
 
 	// How long a cached thread is considered fresh, based on the post's age.
 	// Posts older than FREEZE_AGE are never re-fetched.