feat(notifications): add event batching, localization, digests, and v…

[Osuochasam]

Overview
This PR introduces major architectural enhancements to the notification and webhook delivery pipeline. These updates improve system scalability under high-volume event loads, internationalize user-facing messaging, support flexible digest scheduling, and establish strict version contracts for external webhook consumers.

Key Changes

High-Volume Event Batching:

Implemented collection window and batch size configurations to group high-volume events into single payloads while preserving individual event reference traceability.

Localization & Template Separation:

Extracted user-facing notification text from business logic into locale-specific template files.

Added predictable fallback mechanisms for missing translations and validation tests across supported locales.

Periodic Digest Summaries:

Added configurable scheduling (e.g., hourly, daily) for users opting into periodic activity summaries instead of immediate alerts.

Added robust test suites covering scheduler logic, event accumulation, and digest generation.

Webhook Payload Versioning:

Included explicit version identifiers in outbound webhook payloads and documented compatibility rules to ensure reliable API versioning for external consumers.

Closes #958
Closes #960
Closes #959
Closes #961

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