feat(gcp): add GCP Terraform deployment and Orion build execution environment

[WiedersehenM]

Summary

This PR introduces a GCP Terraform deployment for Mega and provides a GKE-based build execution environment compatible with the existing Mega / Orion-server scheduling model. It adds multi-environment Terraform entrypoints, GCP modules, Orion Worker deployment via Terraform, and E2E validation assets.

Closes/Relates to:

• #1692 (GCP Terraform deployment)
• #1841 (GCP build execution environment)

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What’s Included

GCP Terraform environments

• deployment/envs/gcp/dev|staging|prod
  • Standardized entrypoints (main.tf, providers.tf, versions.tf, variables.tf, terraform.tfvars.example)
  • Outputs aligned with existing tooling where applicable (pg_endpoint, valkey_endpoint, alb_dns_name, etc.)
  • Added project_id output for validation tooling

GCP Terraform modules

• Network: VPC/Subnet, Cloud Router/NAT, firewall rules
• GKE: Cluster + dedicated node pool for build workloads
• Artifact Registry, GCS, Cloud SQL, Memorystore Redis, Filestore
• IAM: service accounts + role bindings + Workload Identity bindings
• Monitoring/Logging: enable required services and optional alerting primitives

Orion build execution environment (GKE)

• Orion Worker deployed via Terraform as a DaemonSet on a dedicated node pool
• Node pool isolation via taints/tolerations and nodeSelector
• HostPath caches for build workspace/data (/workspace, /data) to support build performance and reuse

E2E / validation

• deployment/gcp/e2e/README.e2e.md
• Connectivity validation job (DNS/HTTPS/WS)
• Task trigger & status polling job (Orion-server API)
• Minimal validation script for quick environment checks

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Key Fixes / Compatibility Notes

• Updated Kubernetes resources to provider-supported resource types and v1 resources where required.
• Fixed Terraform heredoc placeholders to avoid HCL interpolation conflicts.
• Fixed nodepool service account handling to avoid coalesce() empty-args failures when no SA is explicitly provided.
• Adjusted IAM example naming to comply with GCP service account account_id constraints (no underscores).
• Updated E2E jobs to avoid hard-coding nodepool names where possible.

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