feat: add onboarding, branding, and supporting data models, migrations, and schemas

Add onboarding_dismissed and branding columns (logo_data, logo_content_type,
company_display_name) to users and teams models. Create SessionSupportingData
model for attaching text snippets and screenshots to sessions. Add Pydantic
schemas for onboarding status, branding responses, and supporting data CRUD.
Update SessionExport to accept pdf format.

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

backend/app/models/team.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import datetime, timezone
-from typing import TYPE_CHECKING
-from sqlalchemy import String, DateTime
+from typing import Optional, TYPE_CHECKING
+from sqlalchemy import String, DateTime, Text
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID
 from app.core.database import Base
@@ -23,6 +23,12 @@ class Team(Base):
         default=uuid.uuid4
     )
     name: Mapped[str] = mapped_column(String(255), nullable=False)
+
+    # Branding
+    logo_data: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    logo_content_type: Mapped[Optional[str]] = mapped_column(String(50), nullable=True)
+    company_display_name: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc)