feat: add featured/locked columns and auto-rotation logic to news pipeline

- Add `ai_grade`, `featured`, `featured_locked`, `landing_locked`, `author`, `article_published`, `image_url`, `has_unique_image` columns to news table
- Extend `news_images` schema with `alt_text`, `phash`, `width`, `height`, `is_placeholder` columns
- Create `idx_news_featured` index for efficient featured queries
- Update admin toggle endpoints to set `featured_locked`/`landing_locked` when manually toggling
- Propagate `featured` and `image_url` fields through landing page, news list, and detail page rendering
- Update `get_featured_news` to return `featured` and `image_url` fields
- Document in AGENTS.md the full zero-maintenance pipeline: image perceptual hashing, placeholder detection, AI grading on cleaned text, reliability gate, effective score computation, and post-loop landing rotation
- Update README.md service description to reflect automatic image comparison and landing rotation capabilities
- Clarify docs_api.py summaries that toggling featured/landing now locks articles from auto-rotation

tests/e2e/admin/news.py

@@ -3,7 +3,7 @@
 from uuid import uuid4
 from datetime import datetime, timezone
 from tests.conftest import BASE_URL
-from devplacepy.database import get_table
+from devplacepy.database import get_table, refresh_snapshot
 from devplacepy.utils import make_combined_slug
 def seed_admin_news(count=3):
     news_table = get_table("news")
@@ -103,6 +103,19 @@ def test_admin_news_landing_toggle(alice):
         page.locator(toggle_sel).first.get_attribute("class") or ""
     )
     assert is_active != was_active
+    refresh_snapshot()
+    assert get_table("news").count(landing_locked=1) >= 1
+
+
+def test_admin_news_featured_locks_row(alice):
+    page, _ = alice
+    seed_admin_news()
+    page.goto(f"{BASE_URL}/admin/news", wait_until="domcontentloaded")
+    toggle_sel = "form[action$='/toggle'] button.admin-toggle-switch"
+    page.locator(toggle_sel).first.click()
+    page.wait_for_url(f"{BASE_URL}/admin/news", wait_until="domcontentloaded")
+    refresh_snapshot()
+    assert get_table("news").count(featured_locked=1) >= 1
 
 
 def test_admin_news_delete(alice):