feat(enterprise): add custom branding system — logo, accent color, company name

- Add branding_logo_url, branding_primary_color, branding_company_name columns to Account model
- Add Alembic migration (58e3f27f3e8f) for branding and SSO columns
- Add GET/PATCH /accounts/me/branding endpoints (owner-only for PATCH)
- Add BrandingSettingsPage with logo URL input, color picker, preview section
- Add /account/branding route (ProtectedRoute owner) in router.tsx
- Add Branding link card in AccountSettingsPage

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

backend/app/models/account.py

@@ -3,7 +3,7 @@ from datetime import datetime, timezone
 from typing import Optional, TYPE_CHECKING
 from sqlalchemy import String, DateTime, ForeignKey, Boolean, Integer
 from sqlalchemy.orm import Mapped, mapped_column, relationship
-from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.core.database import Base
 
 if TYPE_CHECKING:
@@ -44,6 +44,16 @@ class Account(Base):
         Integer, nullable=True, default=100, server_default="100"
     )
 
+    # Custom branding (Task 9)
+    branding_logo_url: Mapped[Optional[str]] = mapped_column(String(500), nullable=True)
+    branding_primary_color: Mapped[Optional[str]] = mapped_column(String(7), nullable=True)  # hex like #06b6d4
+    branding_company_name: Mapped[Optional[str]] = mapped_column(String(200), nullable=True)
+
+    # SSO / SAML groundwork (Task 11)
+    sso_enabled: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
+    sso_provider: Mapped[Optional[str]] = mapped_column(String(20), nullable=True)  # "saml" | "oidc"
+    sso_config: Mapped[Optional[dict]] = mapped_column(JSONB, nullable=True)
+
     # Relationships
     owner: Mapped["User"] = relationship("User", foreign_keys=[owner_id], back_populates="owned_account")
     users: Mapped[list["User"]] = relationship("User", foreign_keys="[User.account_id]", back_populates="account")