feat: implement super admin and platform admin roles

Add multi-platform admin authorization system with:
- AdminPlatform junction table for admin-platform assignments
- is_super_admin flag on User model for global admin access
- Platform selection flow for platform admins after login
- JWT token updates to include platform context
- New API endpoints for admin user management (super admin only)
- Auth dependencies for super admin and platform access checks

Includes comprehensive test coverage:
- Unit tests for AdminPlatform model and User admin methods
- Unit tests for AdminPlatformService operations
- Integration tests for admin users API endpoints

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/database/platform.py

@@ -192,6 +192,13 @@ class Platform(Base, TimestampMixin):
         foreign_keys="SubscriptionTier.platform_id",
     )
 
+    # Admin assignments for this platform
+    admin_platforms = relationship(
+        "AdminPlatform",
+        back_populates="platform",
+        cascade="all, delete-orphan",
+    )
+
     # ========================================================================
     # Indexes
     # ========================================================================