add privacy-first analytics with progressive event collection

Three-layer pipeline: Plug for all HTTP requests (no JS needed), LiveView
hook for SPA navigations, JS hook for screen width. ETS-backed buffer
batches writes to SQLite every 10s. Daily-rotating salt for visitor hashing.
Includes admin dashboard with date ranges, visitor trends, top pages,
sources, devices, and e-commerce conversion funnel. Oban cron for 12-month
data retention.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

priv/repo/migrations/20260222112942_create_analytics_events.exs

@@ -0,0 +1,30 @@
+defmodule Berrypod.Repo.Migrations.CreateAnalyticsEvents do
+  use Ecto.Migration
+
+  def change do
+    create table(:analytics_events, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :name, :string, null: false
+      add :pathname, :string, null: false
+      add :visitor_hash, :binary, null: false
+      add :session_hash, :binary, null: false
+      add :referrer, :string
+      add :referrer_source, :string
+      add :utm_source, :string
+      add :utm_medium, :string
+      add :utm_campaign, :string
+      add :country_code, :string, size: 2
+      add :screen_size, :string
+      add :browser, :string
+      add :os, :string
+      add :revenue, :integer
+
+      timestamps(type: :utc_datetime, updated_at: false)
+    end
+
+    create index(:analytics_events, [:inserted_at])
+    create index(:analytics_events, [:name, :inserted_at])
+    create index(:analytics_events, [:session_hash, :inserted_at])
+    create index(:analytics_events, [:pathname, :inserted_at])
+  end
+end