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

devplacepy/routers/admin/news.py

@@ -85,7 +85,9 @@ async def admin_news_toggle(request: Request, uid: str):
     if article:
         current = article.get("featured", 0)
         new_value = 0 if current else 1
-        news_table.update({"uid": uid, "featured": new_value}, ["uid"])
+        news_table.update(
+            {"uid": uid, "featured": new_value, "featured_locked": 1}, ["uid"]
+        )
         logger.info(
             f"Admin {admin['username']} toggled news {uid} featured={'off' if current else 'on'}"
         )
@@ -137,7 +139,9 @@ async def admin_news_landing(request: Request, uid: str):
     if article:
         current = article.get("show_on_landing", 0)
         new_value = 0 if current else 1
-        news_table.update({"uid": uid, "show_on_landing": new_value}, ["uid"])
+        news_table.update(
+            {"uid": uid, "show_on_landing": new_value, "landing_locked": 1}, ["uid"]
+        )
         logger.info(
             f"Admin {admin['username']} toggled news {uid} landing={'on' if not current else 'off'}"
         )