feat: add captain usage table

db/migrate/20260630102817_create_captain_usages.rb

@@ -0,0 +1,20 @@
+class CreateCaptainUsages < ActiveRecord::Migration[7.1]
+  def change
+    create_table :captain_usages do |t|
+      t.references :account, null: false
+      t.references :assistant, null: false
+      t.integer :usage_type, null: false
+      # Usage is binned into 15-minute UTC buckets so it can be re-sliced into
+      # any caller timezone (including :45 offset zones). Do not widen to hourly.
+      t.datetime :bucket_started_at, null: false
+      t.integer :credits_used, null: false, default: 0
+
+      t.timestamps
+    end
+
+    add_index :captain_usages,
+              [:account_id, :assistant_id, :usage_type, :bucket_started_at],
+              unique: true,
+              name: 'index_captain_usages_unique_bucket'
+  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_06_20_000000) do
+ActiveRecord::Schema[7.1].define(version: 2026_06_30_102817) do
   # These extensions should be enabled to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -431,6 +431,19 @@ ActiveRecord::Schema[7.1].define(version: 2026_06_20_000000) do
     t.index ["enabled"], name: "index_captain_scenarios_on_enabled"
   end
 
+  create_table "captain_usages", force: :cascade do |t|
+    t.bigint "account_id", null: false
+    t.bigint "assistant_id", null: false
+    t.integer "usage_type", null: false
+    t.datetime "bucket_started_at", null: false
+    t.integer "credits_used", default: 0, null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["account_id", "assistant_id", "usage_type", "bucket_started_at"], name: "index_captain_usages_unique_bucket", unique: true
+    t.index ["account_id"], name: "index_captain_usages_on_account_id"
+    t.index ["assistant_id"], name: "index_captain_usages_on_assistant_id"
+  end
+
   create_table "categories", force: :cascade do |t|
     t.integer "account_id", null: false
     t.integer "portal_id", null: false