feat(article): expose auto_publish_platforms derived from publish accounts

Replace the freeform `platforms` field on articles/tasks (parsed out of
wizard_state/input_params JSON) with `auto_publish_platforms`, resolved
by joining schedule publish accounts to platform_accounts. The migration
strips the legacy keys from existing rows so the new field is the single
source of truth.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/migrations/20260501103000_remove_legacy_article_platform_json.down.sql

@@ -0,0 +1 @@
+-- Legacy article target platform JSON keys are intentionally not restored.

server/migrations/20260501103000_remove_legacy_article_platform_json.up.sql

@@ -0,0 +1,11 @@
+UPDATE articles
+SET wizard_state_json = wizard_state_json - 'platforms' - 'target_platforms' - 'target_platform',
+    updated_at = NOW()
+WHERE wizard_state_json IS NOT NULL
+  AND wizard_state_json ?| ARRAY['platforms', 'target_platforms', 'target_platform'];
+
+UPDATE generation_tasks
+SET input_params_json = input_params_json - 'platforms' - 'target_platforms' - 'target_platform',
+    updated_at = NOW()
+WHERE input_params_json IS NOT NULL
+  AND input_params_json ?| ARRAY['platforms', 'target_platforms', 'target_platform'];