Replace scs postgresstore with a sqlc-backed SessionStore.

Keep scs for cookies and session API while sessions DDL and queries live in the same sqlc stack as the rest of Postgres.

db/queries/sessions.sql

@@ -0,0 +1,18 @@
+-- name: GetSession :one
+SELECT data
+FROM sessions
+WHERE token = $1 AND expiry > now();
+
+-- name: UpsertSession :exec
+INSERT INTO sessions (token, data, expiry)
+VALUES ($1, $2, $3)
+ON CONFLICT (token) DO UPDATE
+SET data = excluded.data, expiry = excluded.expiry;
+
+-- name: DeleteSession :exec
+DELETE FROM sessions
+WHERE token = $1;
+
+-- name: DeleteExpiredSessions :exec
+DELETE FROM sessions
+WHERE expiry <= now();