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.

[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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