feat(analytics): add customer daily metrics data foundation

Add the migration + model for per-customer-per-day conversation analytics
(M2 phase 1b). Complements the per-account ConversationDailyMetric with a
per-contact/day view so reports can drill into individual customers and see
per-agent attribution, while the per-account table keeps the aggregate totals.

- customer_daily_metrics: unique (account_id, contact_id, date); message/
  conversation/resolved/unresolved counts, avg session duration, top tags,
  deal outcome breakdown, and agent_ids (jsonb).
- CustomerDailyMetric: belongs_to :account + :contact (Contact convention),
  validations, JSONB getters defaulting to []/{}.

Approved by independent five-key pre-commit review deleg_434fc544
(passed=true, blocking arrays empty).

app/models/customer_daily_metric.rb

@@ -0,0 +1,41 @@
+# == Schema Information
+#
+# Table name: customer_daily_metrics
+#
+#  id                           :bigint           not null, primary key
+#  account_id                   :bigint           not null
+#  contact_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
+#  deal_outcomes                :jsonb            default({}), not null
+#  agent_ids                    :jsonb            default([]), not null
+#  created_at                   :datetime         not null
+#  updated_at                   :datetime         not null
+#
+class CustomerDailyMetric < ApplicationRecord
+  belongs_to :account
+  belongs_to :contact
+
+  validates :account_id, presence: true
+  validates :contact_id, presence: true
+  validates :date, presence: true
+  validates :date, uniqueness: { scope: %i[account_id contact_id] }
+
+  def top_tags
+    self[:top_tags] || []
+  end
+
+  def deal_outcomes
+    self[:deal_outcomes] || {}
+  end
+
+  def agent_ids
+    self[:agent_ids] || []
+  end
+end