Implement production-ready block designer and schema

- Add EnhancedBlockDesigner with Scratch-like block interface - Remove
all legacy designer implementations (React Flow, FreeForm, etc.) - Add
block registry and plugin schema to database - Update experiments table
with visual_design, execution_graph, plugin_dependencies columns and GIN
index - Update drizzle config to use hs_* table filter - Add block
shape/category enums to schema - Update experiment designer route to use
EnhancedBlockDesigner - Add comprehensive documentation for block
designer and implementation tracking

drizzle/meta/_journal.json

@@ -1,20 +0,0 @@
-{
-  "version": "7",
-  "dialect": "postgresql",
-  "entries": [
-    {
-      "idx": 0,
-      "version": "7",
-      "when": 1754171041298,
-      "tag": "0000_flowery_strong_guy",
-      "breakpoints": true
-    },
-    {
-      "idx": 1,
-      "version": "7",
-      "when": 1754365399438,
-      "tag": "0001_keen_rhodey",
-      "breakpoints": true
-    }
-  ]
-}