refactor: normalize script_builder_messages into separate table

Extract JSONB messages array from script_builder_sessions into a proper
script_builder_messages table with individual columns for role, content,
script, tokens, etc. Migration handles data migration from JSONB to rows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/ScriptBuilderPage.tsx

@@ -66,7 +66,7 @@ export default function ScriptBuilderPage() {
     const userMessage: ScriptBuilderMessage = {
       role: 'user',
       content,
-      timestamp: new Date().toISOString(),
+      created_at: new Date().toISOString(),
     }
     setMessages((prev) => [...prev, userMessage])
     setIsLoading(true)
@@ -88,7 +88,7 @@ export default function ScriptBuilderPage() {
         script: response.script,
         script_filename: response.script_filename,
         line_count: response.line_count,
-        timestamp: response.timestamp,
+        created_at: response.timestamp,
       }
       setMessages((prev) => [...prev, assistantMessage])
     } catch (err) {
@@ -96,7 +96,7 @@ export default function ScriptBuilderPage() {
       const errorMessage: ScriptBuilderMessage = {
         role: 'assistant',
         content: `An error occurred: ${err instanceof Error ? err.message : 'Failed to generate response. Please try again.'}`,
-        timestamp: new Date().toISOString(),
+        created_at: new Date().toISOString(),
       }
       setMessages((prev) => [...prev, errorMessage])
     } finally {