add school houses

[chamals3n4]

Description

Adds a Houses feature for sports and inter-house events (e.g. Vijaya, Thissa, Gamunu, Parakum). Every student belongs to one house, decided automatically from their index number.

• Admin manages houses in a Settings tab (name, optional code/colour, and a remainder value).
• On student creation, the house is set by index number mod (number of houses), matched to the house with that remainder, the admin defines the mapping.
• A re-assign button fills in houses for students added earlier, and an admin can still change a student's house manually.
• House is shown on the student list with a column and filter.

Type of change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe above)

Affected areas

• Backend (backend/)
• Frontend (frontend/)
• Database migrations (backend/db/migrations/)

How was this tested?

• go build ./... - passes
• pnpm lint and pnpm build - pass
• Manual: created houses with remainders in Settings, added students and confirmed they were assigned by index number, used the re-assign button for older students, changed a student's house manually, and filtered the student list by house.

Checklist

• go build ./... and go test ./... pass (if backend changed)
• pnpm lint and pnpm build pass (if frontend changed)
• Ran sqlc generate after editing queries/migrations (if applicable)
• No generated files in backend/db/sqlc/ were hand-edited