Unify API and Web routes for maintainability

[AishaA-cpu]

Problem

There are duplicate routers for web and API (e.g., auth.py + web_auth.py, instructor_assignments.py + web_instructor_assignments.py, etc.), resulting in repeated logic, higher maintenance costs, and increased risk of inconsistent behavior.

Proposal

• Refactor routers to use content negotiation, serving both JSON and HTML responses from unified endpoints
• Use FastAPI's JSONResponse and HTMLResponse based on the Accept header
• Remove duplicate router files and consolidate handlers
• Add tests to ensure parity between API and web outputs

Rationale

• Reduces code duplication by 40%+
• Ensures new features, bugfixes, and validation flow consistently to both Web and REST clients
• Easier onboarding for future maintainers

Acceptance Criteria

• No duplicate router files or logic blocks remain
• All existing routes are covered by appropriate tests
• Both JSON and HTML views produce correct results from single handler logic

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Labels: architecture, refactor, api, backend