feat: external_services table create

sql/migrations/000001_init.up.sql

@@ -87,8 +87,13 @@ CREATE TABLE signals (
 
 CREATE TABLE external_ids (
     user_id                                 NOT NULL REFERENCES users (id),
-    service_id          text                NOT NULL,
+    service_id          bigint              REFERENCES external_services (id),
-    external_ids        text                NOT NULL
+    external_id         text                NOT NULL
+);
+
+CREATE TABLE external_services (
+    id                  bigint              GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
+    name                text                UNIQUE NOT NULL
 );
 
 -- Functions