Installed apps’ notification catalogue

For a channel provider: every notification key the apps installed on this tenant declare — names, `variables` (name → type), the developer’s default template and channel ceiling — plus the tenant’s state for the key when it has one (`enabled`, `channels`, `template_id`). These are the keys you will see in `metadata.notification_key`; the variables are what fills your template’s slots. This tenant’s apps only; no recipients, no payloads. `installed_apps` lists every active install with its notification count so an empty answer reads: installs without notifications, or a token on another tenant (`tenant_id`).

• Rate Limit: LIGHT
  How heavily this call counts against your quota: LIGHT < MODERATE < HEAVY.
• Permission needed: notifications.providers.send
  The scope the tenant must grant your app before this call is allowed.
• Reference Code: srv_b2fa
  A stable identifier for this endpoint — quote it to support to speed up tracing.

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