sqldb: add table to track custom SQL migrations

This commit adds the migration_tracker table which we'll use to track if
a custom migration has already been done.

sqldb/sqlc/migrations/000005_migration_tracker.up.sql

@@ -0,0 +1,17 @@
+-- The migration_tracker table keeps track of migrations that have been applied
+-- to the database. This table ensures that migrations are idempotent and are
+-- only run once. It tracks a global database version that encompasses both
+-- schema migrations handled by golang-migrate and custom in-code migrations
+-- for more complex data conversions that cannot be expressed in pure SQL.
+CREATE TABLE IF NOT EXISTS migration_tracker (
+        -- version is the global version of the migration. Note that we
+        -- intentionally don't set it as PRIMARY KEY as it'd auto increment on
+        -- SQLite and our sqlc workflow will replace it with an auto
+        -- incrementing SERIAL on Postgres too. UNIQUE achieves the same effect
+        -- without the auto increment.
+        version INTEGER UNIQUE NOT NULL,
+
+        -- migration_time is the timestamp at which the migration was run.
+        migration_time TIMESTAMP NOT NULL
+);
+