fix(installation_config): normalize legacy native-hash rows to YAML strings

Some rows in installation_configs.serialized_value were written as native
jsonb objects by older code paths, while the YAML coder expects JSON-encoded
YAML strings. Reading the object-shaped rows raised "TypeError: no implicit
conversion of Hash into String" in production after the upstream 4.13.0 merge.

Convert every object-shaped row to the YAML-string shape the coder produces,
so the stock serialize :serialized_value, coder: YAML, ... works for all
rows without needing a custom coder.

db/migrate/20260418020000_normalize_installation_config_serialized_values.rb

@@ -0,0 +1,25 @@
+class NormalizeInstallationConfigSerializedValues < ActiveRecord::Migration[7.1]
+  # Rails' YAML coder expects the jsonb column to hold a JSON-encoded YAML
+  # string. Some rows were written as native jsonb objects by older code paths,
+  # which raises "no implicit conversion of Hash into String" on read. Convert
+  # every `object`-shaped row to the YAML-string shape the coder produces.
+  def up
+    rows = execute(
+      "SELECT id, serialized_value FROM installation_configs WHERE jsonb_typeof(serialized_value) = 'object'"
+    ).to_a
+
+    rows.each do |row|
+      hash = JSON.parse(row['serialized_value']).with_indifferent_access
+      yaml = YAML.dump(hash)
+      execute(
+        ActiveRecord::Base.sanitize_sql_array(
+          ['UPDATE installation_configs SET serialized_value = to_jsonb(?::text) WHERE id = ?', yaml, row['id']]
+        )
+      )
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2026_04_10_170003) do
+ActiveRecord::Schema[7.1].define(version: 2026_04_18_020000) do
   # These extensions should be enabled to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"