Make target connection limits configurable

[subotac]

Description

Makes target PostgreSQL connection-pool limits configurable through target.postgres.max_connections and PGSTREAM_POSTGRES_WRITER_MAX_CONNECTIONS, while retaining the existing default of 50.

The target URL's pool_max_conns value is now honored when no explicit pgstream setting is provided. The resolved limit is applied to the writer and schema-observer pools, and the bulk-ingest COPY budget follows the writer-pool limit.

Related Issue(s)

• Closes Make number of target connections configurable #1047

Type of Change

• New feature
• Documentation update
• Test coverage improvement

Changes Made

• Added YAML and environment configuration for target pool limits.
• Preserved URL configuration and defined explicit configuration precedence.
• Added regression coverage for pool sizing and COPY-budget calculation.

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• Relevant package and non-integration test suites pass

Checklist

• Code follows project style guidelines
• Self-review completed
• Code is well-commented
• Documentation updated where necessary

Additional Notes

The race-enabled and Docker-backed suites were unavailable in the local Windows environment and will be covered by CI.

[github-actions]

Coverage

Total: 59.1% (+0.5% vs main)

Coverage in packages changed by this PR:

Package	Coverage	Δ
cmd/config	84.8%	±0.0%
internal/postgres	45.6%	+2.6%
pkg/wal/processor/postgres	84.2%	+4.3%