fix(db): cascade soft-delete tasks and events on session delete

[mesutoezdil]

deleting a session left related task, event, and session_share rows untouched. this wraps DeleteSession in a transaction that soft-deletes tasks and events and hard-deletes session shares before marking the session itself as deleted.

also adds a migration making session id unique among live sessions. ids were only unique per user, so two users could hold the same session id and deleting one would have cascaded into the other user's tasks and events.

[Copilot]

Pull request overview

This PR updates the Postgres deletion path for sessions so that removing a session also cleans up related data, instead of leaving orphaned task/event/share rows.

Changes:

• Add sqlc queries to soft-delete all tasks and events for a session.
• Add a sqlc query to hard-delete all session shares for a session.
• Wrap DeleteSession in a transaction to execute the cascade cleanup and then soft-delete the session row.

Reviewed changes

Copilot reviewed 4 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
go/core/internal/database/queries/tasks.sql	Adds SoftDeleteTasksBySession SQL query.
go/core/internal/database/queries/session_shares.sql	Adds DeleteSessionSharesBySession SQL query.
go/core/internal/database/queries/events.sql	Adds SoftDeleteEventsBySession SQL query.
go/core/internal/database/gen/tasks.sql.go	Adds generated Go method for SoftDeleteTasksBySession.
go/core/internal/database/gen/session_shares.sql.go	Adds generated Go method for DeleteSessionSharesBySession.
go/core/internal/database/gen/querier.go	Extends the sqlc Querier interface with the new methods.
go/core/internal/database/gen/events.sql.go	Adds generated Go method for SoftDeleteEventsBySession.
go/core/internal/database/client_postgres.go	Wraps DeleteSession in a transaction and invokes the new cascade operations.

Files not reviewed (4)

• go/core/internal/database/gen/events.sql.go: Generated file
• go/core/internal/database/gen/querier.go: Generated file
• go/core/internal/database/gen/session_shares.sql.go: Generated file
• go/core/internal/database/gen/tasks.sql.go: Generated file

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