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/models/script_builder_session.py

@@ -1,14 +1,14 @@
-"""Script Builder session model.
+"""Script Builder session and message models.
 
 Tracks AI-powered script generation conversations.
 """
 import uuid
 from datetime import datetime, timezone
-from typing import Optional, Any, TYPE_CHECKING
+from typing import Optional, TYPE_CHECKING
 
 from sqlalchemy import String, Text, DateTime, ForeignKey, Integer
 from sqlalchemy.orm import Mapped, mapped_column, relationship
-from sqlalchemy.dialects.postgresql import UUID, JSONB
+from sqlalchemy.dialects.postgresql import UUID
 
 from app.core.database import Base
 
@@ -42,10 +42,6 @@ class ScriptBuilderSession(Base):
         String(200), nullable=True,
         comment="Auto-generated from first AI response",
     )
-    messages: Mapped[list[dict[str, Any]]] = mapped_column(
-        JSONB, nullable=False, default=list,
-        comment="Array of {role, content, script?, script_filename?, timestamp}",
-    )
     latest_script: Mapped[Optional[str]] = mapped_column(
         Text, nullable=True,
         comment="Most recent generated script for quick access",
@@ -54,9 +50,6 @@ class ScriptBuilderSession(Base):
         String(200), nullable=True,
         comment="Filename of the latest generated script",
     )
-    message_count: Mapped[int] = mapped_column(
-        Integer, nullable=False, default=0,
-    )
     ai_session_id: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("ai_sessions.id", ondelete="SET NULL"),
@@ -74,3 +67,45 @@ class ScriptBuilderSession(Base):
 
     # Relationships
     user: Mapped["User"] = relationship("User")
+    message_records: Mapped[list["ScriptBuilderMessage"]] = relationship(
+        "ScriptBuilderMessage",
+        back_populates="session",
+        order_by="ScriptBuilderMessage.created_at",
+        cascade="all, delete-orphan",
+    )
+
+
+class ScriptBuilderMessage(Base):
+    """A single message in a Script Builder conversation."""
+    __tablename__ = "script_builder_messages"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    session_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("script_builder_sessions.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    role: Mapped[str] = mapped_column(
+        String(20), nullable=False, comment="user or assistant"
+    )
+    content: Mapped[str] = mapped_column(Text, nullable=False)
+    script: Mapped[Optional[str]] = mapped_column(
+        Text, nullable=True, comment="Extracted script from AI response"
+    )
+    script_filename: Mapped[Optional[str]] = mapped_column(
+        String(200), nullable=True
+    )
+    line_count: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
+    input_tokens: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
+    output_tokens: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+
+    # Relationship
+    session: Mapped["ScriptBuilderSession"] = relationship(
+        "ScriptBuilderSession", back_populates="message_records"
+    )