fix(mastra): persist observability logs in postgres

[tataihono]

Summary

• add a Postgres-backed Mastra observability log store for batchCreateLogs/listLogs
• keep @mastra/pg for traces/spans while storing log records in mastra.mastra_logs
• wire the custom observability domain into the Mastra composite store

Validation

• pnpm --filter @forge/mastra test
• pnpm --filter @forge/mastra typecheck
• pnpm --filter @forge/mastra lint
• pnpm --filter @forge/mastra build
• git diff --check

[railway-app]

🚅 Deployed to the forge-pr-1001 environment in forge

10 services not affected by this PR

• @forge/web
• @forge/admin/redis
• @forge/cms/db
• @forge/admin/db
• @forge/roadmap
• @forge/cms
• @forge/agentic-studio
• @forge/agentic
• @forge/admin
• @forge/manager