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/schemas/onboarding.py

@@ -0,0 +1,12 @@
+from pydantic import BaseModel
+
+
+class OnboardingStatus(BaseModel):
+    created_flow: bool
+    ran_session: bool
+    exported_session: bool
+    tried_ai_assistant: bool
+    invited_teammate: bool
+    connected_psa: bool
+    is_team_user: bool
+    dismissed: bool