feat(pgbench): add initial containerized benchmark runner

[amh1k]

Summary

Adds a standalone PostgreSQL benchmark runner using Go and pgbench.

Fixes #10

Changes

• Added a Go runner with configuration validation, optional table initialization, and default read/write benchmark execution.
• Added cancellation handling and propagation of pgbench failure exit codes.
• Added a multi-stage Dockerfile containing the compiled runner and PostgreSQL 16 tools, running as a non-root user.
• Added unit tests for configuration, password-file validation, execution ordering, environment forwarding, cancellation, and process exit codes.
• Added runners/pgbench/README.md with build, configuration, local testing, and cleanup instructions.

Configuration

Required environment variables:

• PGHOST: PostgreSQL hostname.
• PGDATABASE: Existing database to benchmark.
• PGUSER: PostgreSQL username.
• PGPASSWORD or PGPASSFILE: Password or a readable password file with owner-only permissions.

Optional settings:

Variable	Default
PGPORT	5432
PGCONNECT_TIMEOUT	10 seconds
BENCH_DURATION	60 seconds
BENCH_CLIENTS	1
BENCH_THREADS	1
BENCH_SCALE	1
BENCH_INITIALIZE	false

BENCH_INITIALIZE=true drops and recreates pgbench tables. Use it only with a dedicated disposable database.

CI

Added a runner job that:

• Runs go vet and race-enabled Go tests.
• Builds the runner image.
• Starts disposable PostgreSQL and runs benchmarks with and without initialization.
• Checks throughput and average latency output.
• Verifies authentication failure and checks that the supplied password is absent from failure output.
• Cleans up the integration-test container, volume, and network.

Validation

Runner tests, Docker image build, and Docker integration checks passed locally.

Scope

Results are printed to container logs. Coordinator integration, Kubernetes Job and Secret creation, result persistence, and UI integration remain follow-up work.

[spron-in]

Nice! LGTM