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/api/endpoints/script_builder.py

@@ -9,10 +9,12 @@ from app.core.database import get_db
 from app.core.rate_limit import limiter
 from app.api.deps import get_current_active_user
 from app.models.user import User
+from app.models.script_builder_session import ScriptBuilderSession
 from app.schemas.script_builder import (
     ScriptBuilderCreateRequest,
     ScriptBuilderMessageRequest,
     ScriptBuilderMessageResponse,
+    ScriptBuilderMessageSchema,
     ScriptBuilderSessionDetail,
     ScriptBuilderSessionSummary,
     SaveToLibraryRequest,
@@ -25,6 +27,39 @@ router = APIRouter(prefix="/scripts/builder", tags=["script-builder"])
 MAX_SESSIONS_PER_USER = 5
 
 
+def _session_to_detail(session: ScriptBuilderSession) -> ScriptBuilderSessionDetail:
+    """Convert a session ORM object (with message_records loaded) to detail schema."""
+    messages = [
+        ScriptBuilderMessageSchema.model_validate(m)
+        for m in session.message_records
+    ]
+    return ScriptBuilderSessionDetail(
+        id=session.id,
+        language=session.language,
+        title=session.title,
+        messages=messages,
+        latest_script=session.latest_script,
+        latest_script_filename=session.latest_script_filename,
+        message_count=len([m for m in messages if m.role == "user"]),
+        ai_session_id=session.ai_session_id,
+        created_at=session.created_at,
+        updated_at=session.updated_at,
+    )
+
+
+def _session_to_summary(session: ScriptBuilderSession) -> ScriptBuilderSessionSummary:
+    """Convert a session ORM object to summary schema (no messages needed)."""
+    return ScriptBuilderSessionSummary(
+        id=session.id,
+        language=session.language,
+        title=session.title,
+        message_count=0,  # Summary doesn't eagerly load messages
+        latest_script_filename=session.latest_script_filename,
+        created_at=session.created_at,
+        updated_at=session.updated_at,
+    )
+
+
 @router.post("/sessions", response_model=ScriptBuilderSessionDetail, status_code=201)
 async def create_session(
     data: ScriptBuilderCreateRequest,
@@ -47,7 +82,9 @@ async def create_session(
         language=data.language,
     )
     await db.commit()
-    return ScriptBuilderSessionDetail.model_validate(session)
+    # Re-fetch with message_records loaded
+    session = await script_builder_service.get_session(db, session.id, current_user.id)
+    return _session_to_detail(session)
 
 
 @router.get("/sessions", response_model=list[ScriptBuilderSessionSummary])
@@ -61,7 +98,7 @@ async def list_sessions(
     sessions = await script_builder_service.list_sessions(
         db=db, user_id=current_user.id, limit=limit, offset=offset
     )
-    return [ScriptBuilderSessionSummary.model_validate(s) for s in sessions]
+    return [_session_to_summary(s) for s in sessions]
 
 
 @router.get("/sessions/{session_id}", response_model=ScriptBuilderSessionDetail)
@@ -74,7 +111,7 @@ async def get_session(
     session = await script_builder_service.get_session(db, session_id, current_user.id)
     if not session:
         raise HTTPException(status_code=404, detail="Session not found")
-    return ScriptBuilderSessionDetail.model_validate(session)
+    return _session_to_detail(session)
 
 
 @router.post(