feat(rollup): add models and write path [1/3] (#13796)

## PR#1: Reporting events rollup — model and write path

Reporting queries currently hit the `reporting_events` table directly.
This works, but the table grows linearly with event volume, and
aggregation queries (counts, averages over date ranges) get
progressively slower as accounts age.

This PR introduces a pre-aggregated `reporting_events_rollups` table
that stores daily per-metric, per-dimension (account/agent/inbox)
totals. The write path is intentionally decoupled from the read path —
rollup rows are written inline from the event listener via upsert, and a
backfill service exists to rebuild historical data from raw events.
Nothing reads from this table yet.

The write path activates when an account has a `reporting_timezone` set
(new account setting). The `reporting_events_rollup` feature flag
controls only the future read path, not writes — so rollup data
accumulates silently once timezone is configured. A `MetricRegistry`
maps raw event names to rollup column semantics in one place, keeping
the write and (future) read paths aligned.

### What changed

- Migration for `reporting_events_rollups` with a unique composite index
for upsert
- `ReportingEventsRollup` model
- `reporting_timezone` account setting with IANA timezone validation
- `MetricRegistry` — single source of truth for event-to-metric mappings
- `RollupService` — real-time upsert from event listener
- `BackfillService` — rebuilds rollups for a given account + date from
raw events
- Rake tasks for interactive backfill and timezone setup
- `reporting_events_rollup` feature flag (disabled by default)

### How to test

1. Set a `reporting_timezone` on an account
(`Account.first.update!(reporting_timezone: 'Asia/Kolkata')`)
2. Resolve a conversation or trigger a first response
3. Check `ReportingEventsRollup.where(account_id: ...)` — rows should
appear
4. Run backfill: `bundle exec rake reporting_events_rollup:backfill` and
verify historical data populates

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

db/migrate/20260211145813_create_reporting_events_rollup.rb

@@ -0,0 +1,37 @@
+class CreateReportingEventsRollup < ActiveRecord::Migration[7.1]
+  def change
+    create_rollups_table
+    add_rollups_indexes
+  end
+
+  private
+
+  def create_rollups_table
+    create_table :reporting_events_rollups do |t|
+      t.integer :account_id, null: false
+      t.date :date, null: false
+      t.string :dimension_type, null: false
+      t.bigint :dimension_id, null: false
+      t.string :metric, null: false
+      t.bigint :count, default: 0, null: false
+      t.float :sum_value, default: 0.0, null: false
+      t.float :sum_value_business_hours, default: 0.0, null: false
+
+      t.timestamps
+    end
+  end
+
+  def add_rollups_indexes
+    add_index :reporting_events_rollups,
+              [:account_id, :date, :dimension_type, :dimension_id, :metric],
+              unique: true, name: 'index_rollup_unique_key'
+
+    add_index :reporting_events_rollups,
+              [:account_id, :metric, :date],
+              name: 'index_rollup_timeseries'
+
+    add_index :reporting_events_rollups,
+              [:account_id, :dimension_type, :date],
+              name: 'index_rollup_summary'
+  end
+end

db/schema.rb

@@ -1124,6 +1124,22 @@ ActiveRecord::Schema[7.1].define(version: 2026_02_26_153427) do
     t.index ["user_id"], name: "index_reporting_events_on_user_id"
   end
 
+  create_table "reporting_events_rollups", force: :cascade do |t|
+    t.integer "account_id", null: false
+    t.date "date", null: false
+    t.string "dimension_type", null: false
+    t.bigint "dimension_id", null: false
+    t.string "metric", null: false
+    t.bigint "count", default: 0, null: false
+    t.float "sum_value", default: 0.0, null: false
+    t.float "sum_value_business_hours", default: 0.0, null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["account_id", "date", "dimension_type", "dimension_id", "metric"], name: "index_rollup_unique_key", unique: true
+    t.index ["account_id", "dimension_type", "date"], name: "index_rollup_summary"
+    t.index ["account_id", "metric", "date"], name: "index_rollup_timeseries"
+  end
+
   create_table "sla_events", force: :cascade do |t|
     t.bigint "applied_sla_id", null: false
     t.bigint "conversation_id", null: false