feat(usage): prompt existing admins once for usage reporting after an update

An admin who already had PicPeak installed before the opt-in reporting
feature existed never gets asked — the setup wizard only runs once, on
a brand-new instance. Adds a one-time modal, shown on the admin's next
dashboard visit after updating, offering the same choice the wizard
gives a new install.

- New `product_usage_state.prompt_shown` column (migration 211) and
  UsageService.markPromptShown(), set on either outcome (enable or
  decline) from both this modal and the wizard step, so an
  installation is never asked twice regardless of which path it took.
- New POST /admin/usage/prompt-seen endpoint.
- Extracted the wizard's three-point pitch (UsageReportingPitch.tsx)
  so the modal and the wizard step share identical copy instead of
  drifting apart.
- The modal never shows once participation is already active, and
  never shows a second time after either the wizard or the modal has
  been through it once.

Depends on #1360 (the setup wizard step this reuses).

backend/migrations/core/211_product_usage_prompt_shown.js

@@ -0,0 +1,25 @@
+// Tracks whether this installation has ever been offered the one-time
+// usage-reporting opt-in prompt shown to an existing admin on their first
+// login after an update (see UsageService.markPromptShown()). A fresh
+// install that went through the setup wizard's own opt-in step sets this
+// too, so upgraded and brand-new installs share one "already asked" marker
+// and neither gets asked twice. Separate from `notice_dismissed`, which
+// governs the persistent, re-visitable dashboard banner instead.
+exports.up = async function (knex) {
+  if (
+    (await knex.schema.hasTable('product_usage_state')) &&
+    !(await knex.schema.hasColumn('product_usage_state', 'prompt_shown'))
+  ) {
+    await knex.schema.alterTable('product_usage_state', (t) => {
+      t.boolean('prompt_shown').notNullable().defaultTo(false);
+    });
+  }
+};
+exports.down = async function (knex) {
+  if (
+    (await knex.schema.hasTable('product_usage_state')) &&
+    (await knex.schema.hasColumn('product_usage_state', 'prompt_shown'))
+  ) {
+    await knex.schema.alterTable('product_usage_state', (t) => t.dropColumn('prompt_shown'));
+  }
+};