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/__init__.py

@@ -39,6 +39,7 @@ from .script_template import ScriptCategory, ScriptTemplate, ScriptGeneration
 from .psa_connection import PsaConnection
 from .psa_post_log import PsaPostLog
 from .psa_member_mapping import PsaMemberMapping
+from .supporting_data import SessionSupportingData
 
 __all__ = [
     "User",
@@ -92,4 +93,5 @@ __all__ = [
     "PsaConnection",
     "PsaPostLog",
     "PsaMemberMapping",
+    "SessionSupportingData",
 ]