templateManager.ts: Add template versioning support

[Kingsman-99]

Description

src/templateManager.ts manages notification and document templates but stores only the latest version of each template. When a template is updated, the previous version is overwritten, making it impossible to roll back or audit changes.

Acceptance Criteria

• Each template is stored with a version number (integer, starting at 1)
• updateTemplate(id, content) increments the version and retains the previous version in history
• getTemplate(id, version?) retrieves a specific version or the latest when version is omitted
• Unit tests pass

Context

• Target file: src/templateManager.ts
• Keep version history in memory; no persistence layer changes are needed