feat: migration 0020 group_key + move state-of-padel to reports/

- Migration 0020: add group_key TEXT column + index to articles table
- DELETE state-of-padel rows from articles (slug collision, moving to reports)
- git mv state-of-padel-q1-2026-{en,de}.md → data/content/reports/

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

web/src/padelnomics/migrations/versions/0020_articles_group_key.py

@@ -0,0 +1,24 @@
+"""Add group_key column to articles for cross-language static article grouping.
+
+Static cornerstone articles (e.g. padel-hall-cost-guide EN vs padel-halle-kosten DE)
+have different url_paths per language, so they cannot be grouped by url_path alone.
+The group_key (populated from the `cornerstone` frontmatter field, e.g. 'C2') is a
+shared key that pairs EN/DE variants of the same static article.
+
+pSEO articles leave group_key NULL and continue to group by url_path (unchanged).
+
+Also removes any state-of-padel articles that are being moved to the reports flow.
+"""
+
+
+def up(conn):
+    conn.execute(
+        "ALTER TABLE articles ADD COLUMN group_key TEXT DEFAULT NULL"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_articles_group_key"
+        " ON articles(group_key)"
+    )
+    # Remove state-of-padel rows — these are moving to the reports flow
+    # and would cause slug collisions (both EN/DE use slug 'state-of-padel-q1-2026')
+    conn.execute("DELETE FROM articles WHERE slug LIKE 'state-of-padel%'")