feat(communications): unified CommunicationTemplate CRUD and orchestration

[kkopanidis]

Problem

Unified CommunicationTemplate documents existed in schema and GitOps export, but operators could not manage them via Admin API or gRPC, and templateName on orchestrated sends was ignored. Email-only templates and multi-channel sends could not share a single resolution path.

Summary

• CommunicationTemplateService with validation, Handlebars resolution, and email-template migration helper
• Admin REST CRUD at /communications/templates plus templateName on /send/multiple, /send/fallback, and new /communications/send
• gRPC CRUD RPCs and real RegisterCommunicationTemplate implementation
• Orchestrator resolves CommunicationTemplate first, EmailTemplate fallback for email (preserves external-managed email path)
• GitOps import validation and email-template name collision checks (priority 30 unchanged)
• communication_templates_total metric and unified send counters on orchestrated paths
• grpc-sdk client methods and optional templateName on send helpers

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other (please describe)

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the main branch

Test plan

• POST /communications/templates — create email+push template; verify variables auto-extracted
• GET/PATCH/DELETE /communications/templates/:id — round-trip CRUD
• POST /communications/send with templateName — single-channel send renders template content
• POST /send/multiple with templateName — per-channel rendered content
• Send with templateName matching only EmailTemplate — email still sends (external-managed path intact)
• Create unified template with same name as existing EmailTemplate — expect ALREADY_EXISTS
• GitOps import communicationTemplates record with invalid channel/content — record fails with descriptive error
• POST /communications/templates/migrate-from-email with dryRun: true — returns planned documents
• gRPC RegisterCommunicationTemplate + ListCommunicationTemplates return persisted JSON