migrate site id to varchar instead of int

backend/database/erd_schema.sql

@@ -10,6 +10,7 @@
 --    is_active true, is_superuser false. Product roles never use is_staff.)
 --
 --   user_access → sites → kandang → cycles
+--                 (sites.site_id = farm slug = active_sites.source_site_id when linked)
 --                 kandang → flock → iot_panel
 --   user_access → api_keys
 --   user_access → active_sites (managed_by)
@@ -103,10 +104,12 @@ CREATE INDEX IF NOT EXISTS idx_active_sites_managed_by_id ON active_sites (manag
 CREATE INDEX IF NOT EXISTS idx_active_sites_source_site_id ON active_sites (source_site_id);
 
 -- -----------------------------------------------------------------------------
--- 4. Sites
+-- 4. Sites (HQ farm tree)
+--    site_id is the farm dashboard Site.site_id slug (e.g. sukawarna-4821).
+--    When linked, it equals active_sites.source_site_id.
 -- -----------------------------------------------------------------------------
 CREATE TABLE IF NOT EXISTS sites (
-    site_id         BIGSERIAL PRIMARY KEY,
+    site_id         VARCHAR(64) PRIMARY KEY,
     site_name       VARCHAR(30) NOT NULL,
     user_id         BIGINT      NOT NULL REFERENCES user_access (user_id) ON DELETE CASCADE,
     active_site_id  BIGINT      UNIQUE REFERENCES active_sites (active_site_id) ON DELETE SET NULL,
@@ -123,7 +126,7 @@ CREATE TABLE IF NOT EXISTS kandang (
     kandang_id           BIGSERIAL PRIMARY KEY,
     kandang_name         VARCHAR(30) NOT NULL,
     feed_in_button_urls  JSONB       NOT NULL DEFAULT '[]'::jsonb,
-    site_id              BIGINT      NOT NULL REFERENCES sites (site_id) ON DELETE CASCADE,
+    site_id              VARCHAR(64) NOT NULL REFERENCES sites (site_id) ON DELETE CASCADE,
     created_at           TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP,
     updated_at           TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP
 );