feat: move article generation to background worker queue

- Add generate_articles task handler to worker.py
- template_generate and template_regenerate now enqueue tasks instead
  of running inline (was blocking HTTP request for seconds with 1k articles)
- rebuild_all enqueues per-template + inline rebuilds manual articles
- Update tests to check task enqueue instead of immediate article creation

Subtask 4 of CMS admin improvement.

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

web/tests/test_content.py

@@ -1019,7 +1019,7 @@ class TestAdminTemplateGenerate:
         assert "3" in html  # 3 rows available
         assert "Generate" in html
 
-    async def test_generate_creates_articles(self, admin_client, db, pseo_env):
+    async def test_generate_enqueues_task(self, admin_client, db, pseo_env):
         async with admin_client.session_transaction() as sess:
             sess["csrf_token"] = "test"
 
@@ -1030,11 +1030,16 @@ class TestAdminTemplateGenerate:
         })
         assert resp.status_code == 302
 
-        articles = await fetch_all("SELECT * FROM articles")
-        assert len(articles) == 3
-
-        scenarios = await fetch_all("SELECT * FROM published_scenarios")
-        assert len(scenarios) == 3
+        # Generation is now queued, not inline
+        tasks = await fetch_all(
+            "SELECT * FROM tasks WHERE task_name = 'generate_articles'"
+        )
+        assert len(tasks) == 1
+        import json
+        payload = json.loads(tasks[0]["payload"])
+        assert payload["template_slug"] == "test-city"
+        assert payload["start_date"] == "2026-04-01"
+        assert payload["articles_per_day"] == 2
 
     async def test_generate_unknown_template_redirects(self, admin_client, db, pseo_env):
         resp = await admin_client.get("/admin/templates/nonexistent/generate")
@@ -1042,13 +1047,7 @@ class TestAdminTemplateGenerate:
 
 
 class TestAdminTemplateRegenerate:
-    async def test_regenerate_updates_articles(self, admin_client, db, pseo_env):
-        from padelnomics.content import generate_articles
-        # First generate
-        await generate_articles("test-city", date(2026, 3, 1), 10)
-        initial = await fetch_all("SELECT * FROM articles")
-        assert len(initial) == 3
-
+    async def test_regenerate_enqueues_task(self, admin_client, db, pseo_env):
         async with admin_client.session_transaction() as sess:
             sess["csrf_token"] = "test"
 
@@ -1057,9 +1056,14 @@ class TestAdminTemplateRegenerate:
         })
         assert resp.status_code == 302
 
-        # Same count — upserted, not duplicated
-        articles = await fetch_all("SELECT * FROM articles")
-        assert len(articles) == 3
+        # Regeneration is now queued, not inline
+        tasks = await fetch_all(
+            "SELECT * FROM tasks WHERE task_name = 'generate_articles'"
+        )
+        assert len(tasks) == 1
+        import json
+        payload = json.loads(tasks[0]["payload"])
+        assert payload["template_slug"] == "test-city"
 
     async def test_regenerate_unknown_template_redirects(self, admin_client, db, pseo_env):
         async with admin_client.session_transaction() as sess: