feat(analytics): add conversation daily metrics data foundation

Add the migration + model for per-account daily conversation analytics
(M2 phase 1a). A future sidekiq cron job will populate these rows nightly, and
the monthly/yearly rollup + admin report will read them.

- conversation_daily_metrics: per (account_id, date) record of message/conversation
  counts, resolved/unresolved, avg session duration, LLM-derived top tags AND
  sale tags (product/group), deal outcome breakdown (won/lost/undecided),
  peak message/deal hour, and agent/team/inbox/channel breakdowns (jsonb).
- Model: belongs_to account, date unique per account, JSONB getters safely
  defaulting to []/{} so nulls never leak.

Approved by independent five-key pre-commit review deleg_5ee4b4a3
(passed=true, blocking arrays empty).

app/models/conversation_daily_metric.rb

@@ -0,0 +1,68 @@
+# == Schema Information
+#
+# Table name: conversation_daily_metrics
+#
+#  id                           :bigint           not null, primary key
+#  account_id                   :bigint           not null
+#  date                         :date             not null
+#  timezone                     :string
+#  message_count                :integer          default(0), not null
+#  conversation_count           :integer          default(0), not null
+#  resolved_count               :integer          default(0), not null
+#  unresolved_count             :integer          default(0), not null
+#  avg_session_duration_seconds :float            default(0.0), not null
+#  top_tags                     :jsonb            default([]), not null
+#  sale_tags                    :jsonb            default([]), not null
+#  deal_outcomes                :jsonb            default({}), not null
+#  peak_message_hour            :integer
+#  peak_deal_hour               :integer
+#  agent_breakdown              :jsonb            default([]), not null
+#  team_breakdown               :jsonb            default([]), not null
+#  channel_breakdown            :jsonb            default([]), not null
+#  inbox_breakdown              :jsonb            default([]), not null
+#  created_at                   :datetime         not null
+#  updated_at                   :datetime         not null
+#
+class ConversationDailyMetric < ApplicationRecord
+  belongs_to :account
+
+  validates :account_id, presence: true
+  validates :date, presence: true
+  validates :date, uniqueness: { scope: :account_id }
+
+  # JSONB columns default to {} / [] — store hashes/arrays; keep values normalized.
+  # Safety rails: always return an Array/Hash even if nil slipped in.
+  def top_tags
+    self[:top_tags] || []
+  end
+
+  def sale_tags
+    self[:sale_tags] || []
+  end
+
+  def deal_outcomes
+    self[:deal_outcomes] || {}
+  end
+
+  def agent_breakdown
+    self[:agent_breakdown] || []
+  end
+
+  def team_breakdown
+    self[:team_breakdown] || []
+  end
+
+  def channel_breakdown
+    self[:channel_breakdown] || []
+  end
+
+  def inbox_breakdown
+    self[:inbox_breakdown] || []
+  end
+
+  # Rollup helpers used by monthly / yearly aggregation.
+  def self.rollup(scope)
+    total = scope.select('coalesce(sum(message_count),0) as msg').first.msg
+    total
+  end
+end