ci: add CI workflow for PRs and main branch

[init4samwise]

Summary

Adds GitHub Actions CI workflow for the blockscout repo.

Changes

New .github/workflows/ci.yml with:

• Compile — Compiles with --warnings-as-errors
• Format — Checks code formatting with mix format --check-formatted
• Credo — Runs linting with mix credo --strict
• Test — Runs tests with PostgreSQL service container

Triggers

• Push to master or main
• PRs targeting master or main

Notes

Uses Elixir 1.19.4 and OTP 27.3.4.6 (matching existing config.yml).

Closes ENG-1913

[init4samwise]

Closing this PR because CI is already enabled via the existing config.yml workflow inherited from upstream Blockscout.

The existing workflow (config.yml) already provides:

• ✅ Compile
• ✅ Format check (mix format --check-formatted)
• ✅ Credo (linting)
• ✅ Dialyzer (type checking)
• ✅ Comprehensive tests (EthereumJSONRPC, Explorer, Indexer, BlockscoutWeb)
• ✅ Additional checks: ESLint, Jest, Sobelow security analysis, spell checking

All these checks are passing on PRs to master. No additional CI workflow is needed.