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>

backend/app/schemas/script_builder.py

@@ -1,6 +1,6 @@
 """Pydantic schemas for the AI Script Builder."""
 from datetime import datetime
-from typing import Any, Optional
+from typing import Optional
 from uuid import UUID
 
 from pydantic import BaseModel, Field
@@ -20,6 +20,21 @@ class ScriptBuilderMessageRequest(BaseModel):
     content: str = Field(min_length=1, max_length=5000)
 
 
+class ScriptBuilderMessageSchema(BaseModel):
+    """A single message in a builder session."""
+    id: UUID
+    role: str
+    content: str
+    script: str | None = None
+    script_filename: str | None = None
+    line_count: int | None = None
+    input_tokens: int | None = None
+    output_tokens: int | None = None
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
 class ScriptBuilderMessageResponse(BaseModel):
     """AI response to a builder message."""
     role: str = "assistant"
@@ -37,7 +52,7 @@ class ScriptBuilderSessionSummary(BaseModel):
     id: UUID
     language: str
     title: str | None = None
-    message_count: int
+    message_count: int = 0
     latest_script_filename: str | None = None
     created_at: datetime
     updated_at: datetime
@@ -50,10 +65,10 @@ class ScriptBuilderSessionDetail(BaseModel):
     id: UUID
     language: str
     title: str | None = None
-    messages: list[dict[str, Any]]
+    messages: list[ScriptBuilderMessageSchema] = []
     latest_script: str | None = None
     latest_script_filename: str | None = None
-    message_count: int
+    message_count: int = 0
     ai_session_id: UUID | None = None
     created_at: datetime
     updated_at: datetime