fix: allow session id reuse after delete

[QuentinBisson]

Fixes #2279.

What

DeleteSession soft-deletes, but UpsertSession's ON CONFLICT update never cleared deleted_at, so recreating a previously deleted session id upserted an invisible row: the create handler's own reload failed with 500 "Failed to load created session", forever. This breaks any client that derives session IDs deterministically from an external key.

UpsertSession now resurrects a soft-deleted (id, user_id) as a fresh incarnation, in one statement:

• clears deleted_at and restarts created_at
• soft-deletes the previous incarnation's events and owned tasks, and drops its shares, so the recreated session starts empty instead of inheriting the old history

NULL-owned tasks are left alone: the created_at restart already hides them from the new incarnation via the ownership bound in tasks.sql, and they may still resolve to another user's same-id session. Upserts of live sessions are unchanged (no purge, created_at preserved).

Tests

TestSessionRecreateAfterDelete (recreate succeeds; events/tasks/shares empty; another user's same-id session untouched) and TestUpsertLiveSessionKeepsHistory (no purge / no created_at reset on live upsert).

[Copilot]

Pull request overview

This PR fixes a correctness issue in the Go/Postgres persistence layer where a session ID could not be reused after a soft delete. It updates the UpsertSession SQL to properly “resurrect” a soft-deleted (id, user_id) session while preventing the recreated session from inheriting prior history.

Changes:

• Update UpsertSession to clear deleted_at, reset created_at only when resurrecting, and purge prior incarnation data (events, owned tasks, shares) in the same statement.
• Regenerate SQLC outputs to reflect the new query and add doc comments on the generated Go interface/methods.
• Add database client tests covering session recreation after delete and ensuring live-session upserts preserve history.

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated no comments.

File	Description
go/core/internal/database/queries/sessions.sql	Adjusts UpsertSession to resurrect soft-deleted sessions and purge prior data atomically.
go/core/internal/database/gen/sessions.sql.go	SQLC-generated Go wrapper updated for the new UpsertSession query and docs.
go/core/internal/database/gen/querier.go	SQLC-generated interface comment updated for UpsertSession.
go/core/internal/database/client_test.go	Adds tests validating recreate-after-delete behavior and no-op behavior for live upserts.

Files not reviewed (2)

• go/core/internal/database/gen/querier.go: Generated file
• go/core/internal/database/gen/sessions.sql.go: Generated file

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[iplay88keys]

Discussed a similar case in this thread, related to resurrecting tasks.