feat: automate GCP explorer deployment

[JOY (JOY)]

Summary

• configure the relaunched DOS Chain mainnet explorer on the GCP VM
• enable Account Abstraction, internal and pending transactions, public metrics, system tags, rate limiting, and SSRF safeguards
• keep shared backend features identical across Mainnet and Testnet, with an automated parity guard
• replace the legacy Azure SSH deployment with GCP Workload Identity Federation and IAP
• update backend containers to the published v11.2.3 image pinned by digest
• update the release workflow so future published images automatically update compose pins and trigger GCP deployment

Environment boundaries

• Mainnet and Testnet share operational and integration feature flags from common-blockscout.env
• Testnet keeps internal transactions, pending transactions, and Account Abstraction disabled because its RPC lacks debug and txpool methods and no verified EntryPoint bytecode is deployed
• chain identity, RPC endpoints, archive support, EntryPoints, and system contract tags remain environment-specific

Validation

• python scripts/validate-blockscout-env-parity.py
• docker compose config -q for Mainnet, Testnet, and Beta
• actionlint for deploy and publish workflows
• git diff --check
• the v11.2.3 image and digest were verified against successful GitHub Actions run 30124157139
• production explorer, Account Abstraction API, public metrics, and User operations page were verified before this workflow change

Deployment

Merging this PR will enable the GCP deployment path. The Deploy Config workflow is currently disabled and will be enabled immediately before merge so the main branch push performs the first automated deployment.

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