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.

schema.sql

@@ -36,3 +36,10 @@ CREATE TABLE IF NOT EXISTS answers (
   created_at TEXT NOT NULL,
   updated_at TEXT NOT NULL
 );
+
+CREATE TABLE IF NOT EXISTS sessions (
+  token TEXT PRIMARY KEY,
+  data BYTEA NOT NULL,
+  expiry TIMESTAMPTZ NOT NULL
+);
+CREATE INDEX IF NOT EXISTS sessions_expiry_idx ON sessions (expiry);