feat: move Captain conversation outcomes to episode grain [CW-7792] (#15315)

Moves Captain conversation outcomes from one row per conversation to one
row per **engagement episode**: a new row each time demand for Captain
(re)starts - first eligible message, a reopen after resolution, or
(reserved) explicit assignment. Each episode has its own demand anchor,
window, and trigger, so returning customers count as new demand and
later cycles can't overwrite an earlier episode's handoff reason,
resolution, or CSAT. Also adds `conversation_outcomes` associations on
Account, Inbox, Conversation, and Captain::Assistant.

## Why this wasn't in #15233

The episode design came out of reviewing the wiring PR: per-conversation
grain couldn't answer per-cycle questions without a patch per field. The
table is unreleased with no writers, so changing the grain now is a pure
schema swap - and landing it first means the tracker gets reviewed
against the final model.

## What changed

- Adds `episode_trigger`, `started_at`, `ended_at`
- Drops `reopen_count` and `last_reopened_at` - reopens are episode rows
now
- Uniqueness moves from `(account, assistant, conversation)` to
per-boundary `(account, conversation, started_at)`
- Two partial unique indexes: one open episode per conversation, one
initial episode per stream
- Model: trigger enum, `started_at` uniqueness validation,
`chronological`/`covering` scopes

enterprise/app/models/captain/assistant.rb

@@ -39,6 +39,7 @@ class Captain::Assistant < ApplicationRecord
   has_many :copilot_threads, dependent: :destroy_async
   has_many :scenarios, class_name: 'Captain::Scenario', dependent: :destroy_async
   has_many :agent_sessions, class_name: 'Captain::AgentSession', dependent: :destroy_async
+  has_many :conversation_outcomes, dependent: :destroy_async
 
   store_accessor :config, :temperature, :feature_faq, :feature_memory, :feature_contact_attributes, :product_name
 

enterprise/app/models/conversation_outcome.rb

@@ -6,14 +6,15 @@
 #  captain_reply_count     :integer          default(0), not null
 #  csat_rating             :integer
 #  csat_received_at        :datetime
+#  ended_at                :datetime
+#  episode_trigger         :string           default("initial"), not null
 #  first_captain_reply_at  :datetime
 #  first_human_reply_at    :datetime
 #  handoff_at              :datetime
 #  handoff_reason_category :string
 #  last_captain_reply_at   :datetime
-#  last_reopened_at        :datetime
-#  reopen_count            :integer          default(0), not null
 #  resolved_at             :datetime
+#  started_at              :datetime         not null
 #  created_at              :datetime         not null
 #  updated_at              :datetime         not null
 #  account_id              :bigint           not null
@@ -23,10 +24,12 @@
 #
 # Indexes
 #
-#  idx_conversation_outcomes_on_assistant_created_at   (account_id,assistant_id,created_at)
+#  idx_conversation_outcomes_initial_episode           (account_id,conversation_id) UNIQUE WHERE ((episode_trigger)::text = 'initial'::text)
 #  idx_conversation_outcomes_on_assistant_handoff_at   (account_id,assistant_id,handoff_at)
 #  idx_conversation_outcomes_on_assistant_resolved_at  (account_id,assistant_id,resolved_at)
-#  idx_conversation_outcomes_unique_conversation       (account_id,assistant_id,conversation_id) UNIQUE
+#  idx_conversation_outcomes_on_assistant_started_at   (account_id,assistant_id,started_at)
+#  idx_conversation_outcomes_open_episode              (account_id,conversation_id) UNIQUE WHERE (ended_at IS NULL)
+#  idx_conversation_outcomes_unique_boundary           (account_id,conversation_id,started_at) UNIQUE
 #  index_conversation_outcomes_on_account_id           (account_id)
 #  index_conversation_outcomes_on_assistant_id         (assistant_id)
 #  index_conversation_outcomes_on_conversation_id      (conversation_id)
@@ -43,6 +46,12 @@ class ConversationOutcome < ApplicationRecord
     usage_limit
   ].freeze
 
+  EPISODE_TRIGGERS = %w[
+    initial
+    reopen
+    assignment
+  ].freeze
+
   belongs_to :account
   belongs_to :assistant, class_name: 'Captain::Assistant'
   belongs_to :conversation, class_name: '::Conversation'
@@ -53,9 +62,22 @@ class ConversationOutcome < ApplicationRecord
        prefix: :handoff_reason,
        validate: { allow_nil: true }
 
-  validates :conversation_id, uniqueness: { scope: [:account_id, :assistant_id] }
+  enum :episode_trigger,
+       EPISODE_TRIGGERS.index_by(&:itself),
+       prefix: :trigger,
+       validate: true
+
+  validates :started_at, presence: true, uniqueness: { scope: [:account_id, :conversation_id] }
   validate :associations_must_belong_to_account
 
+  scope :chronological, -> { order(:started_at) }
+
+  # The half-open episode window [started_at, ended_at); ended_at is nil for
+  # the stream's latest episode.
+  scope :covering, lambda { |at|
+    where(started_at: ..at).where('ended_at IS NULL OR ended_at > ?', at)
+  }
+
   private
 
   def associations_must_belong_to_account

enterprise/app/models/enterprise/concerns/account.rb

@@ -16,6 +16,7 @@ module Enterprise::Concerns::Account
     has_many :captain_documents, dependent: :destroy_async, class_name: 'Captain::Document'
     has_many :captain_custom_tools, dependent: :destroy_async, class_name: 'Captain::CustomTool'
     has_many :captain_agent_sessions, dependent: :destroy_async, class_name: 'Captain::AgentSession'
+    has_many :conversation_outcomes, dependent: :destroy_async
 
     has_many :copilot_threads, dependent: :destroy_async
     has_many :companies, dependent: :destroy_async

enterprise/app/models/enterprise/concerns/conversation.rb

@@ -8,6 +8,7 @@ module Enterprise::Concerns::Conversation
     has_many :calls, dependent: :destroy_async
     has_many :captain_responses, class_name: 'Captain::AssistantResponse', dependent: :nullify, as: :documentable
     has_many :captain_faq_observations, class_name: 'Captain::FaqObservation', dependent: :delete_all
+    has_many :conversation_outcomes, dependent: :destroy_async
     scope :with_sla_applicable_contact, -> { left_joins(:contact).where(contacts: { blocked: [false, nil] }) }
 
     before_validation :validate_sla_policy, if: -> { sla_policy_id_changed? }

enterprise/app/models/enterprise/concerns/inbox.rb

@@ -8,6 +8,7 @@ module Enterprise::Concerns::Inbox
             class_name: 'Captain::Assistant'
     has_many :inbox_capacity_limits, dependent: :destroy
     has_many :calls, dependent: :destroy_async
+    has_many :conversation_outcomes, dependent: :destroy_async
 
     before_create :ensure_create_permitted
   end