Add notifications templates and category-aware routing contract

[haasonsaas]

Summary

• add notification categories, category channel preferences, and template messages/RPCs
• extend SendRequest and Notification with category/template fields
• regenerate Go, Python, and TypeScript notifications bindings

Testing

• buf lint
• go test ./... -count=1
• npm ci
• npm run check:package

[cursor]

PR Summary

Medium Risk
Medium risk because it expands the public notifications proto/API (new fields, messages, and RPCs), requiring coordinated server/client updates and potential backward-compatibility considerations for older clients.

Overview
Adds a new NotificationCategory enum and extends Notification/SendRequest with category plus template-related fields (template_name, template_variables_json) to support category-aware routing and templated notifications.

Introduces per-category channel overrides via CategoryChannelPreference on NotificationPreferences, and adds NotificationTemplate plus new UpsertTemplate and ListTemplates RPCs. Regenerates Go, Python, and TypeScript/Connect generated bindings to reflect the updated proto contract.

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