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).

db/migrate/20260819000001_create_customer_daily_metrics.rb

@@ -0,0 +1,25 @@
+class CreateCustomerDailyMetrics < ActiveRecord::Migration[7.1]
+  def change
+    create_table :customer_daily_metrics do |t|
+      t.bigint :account_id, null: false
+      t.bigint :contact_id, null: false
+      t.date :date, null: false
+      t.string :timezone
+      t.integer :message_count, default: 0, null: false
+      t.integer :conversation_count, default: 0, null: false
+      t.integer :resolved_count, default: 0, null: false
+      t.integer :unresolved_count, default: 0, null: false
+      t.float :avg_session_duration_seconds, default: 0.0, null: false
+      t.jsonb :top_tags, default: [], null: false
+      t.jsonb :deal_outcomes, default: {}, null: false
+      t.jsonb :agent_ids, default: [], null: false
+      t.timestamps
+    end
+
+    add_index :customer_daily_metrics, [:account_id, :contact_id, :date], unique: true,
+              name: 'index_cdm_customer_on_account_contact_date'
+    add_index :customer_daily_metrics, :contact_id
+    add_index :customer_daily_metrics, :account_id
+    add_index :customer_daily_metrics, :date
+  end
+end