feat/auth layer apply roles

[Copilot]

Implements Phase 2 of authentication layer: role-based authorization (user vs admin), creator tracking for proposals and stream analysis, and test fixture updates.

Auth & Authorization

• Added is_admin property to User model based on role field
• Implemented admin_required and login_required decorators for endpoint protection
• Admin endpoints now enforce role-based access control

Data Model Changes

• Added created_by foreign key to proposals and stream_analysis tables
• Established user relationships for tracking resource ownership
• Creator assignment handled in route handlers

Test Infrastructure

• Updated conftest.py with test_user, admin_user, and login helper fixtures
• Adjusted tests to use flush() instead of commit() for proper nested transaction handling
• Fixed fixture parameter passing in unit and integration tests

Code Quality

• Applied black formatting across codebase
• Expanded Phase 2 auth-roles specification documentation

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