feat: move snippets backend and tables into a plugin (B2)

drizzle/sqlite/0030_quick_felicia_hardy.sql

@@ -0,0 +1,4 @@
+-- snippets, snippet_folders and snippet_access moved to the snippets plugin
+-- (p_snippets_*), adopted by rename rather than dropped. A real DROP TABLE
+-- here would run before the plugin's adoption migration and delete every row.
+SELECT 1;