feat(engines): add PostgreSQL driver

[phuthuycoding]

Summary

• New internal/driver/postgres package implementing driver.Driver — postgres:db databases clone between any two endpoints (remote→local, local→remote, remote→remote) with the same streaming, retried, pooled pipeline as mongo/mysql.
• Whole-db clone: pg_dump -Fd writes a transient archive dir inside the local container → pg_restore -j --clean --if-exists --no-owner --no-privileges restores it (pg_restore -j cannot read from stdin, so the stage lives inside the container and is removed per attempt). Dump-side -j is skipped to avoid snapshot privileges on managed hosts.
• -only postgres:db.schema.table: plain-format pg_dump -Fp -t in three phases — pre-data+data, selected tables' trigger functions via pg_get_functiondef, then post-data through an awk filter that drops FKs referencing unselected objects (reported as warnings).
• Local container contract: postgres image, DBCLONE_POSTGRES_CONTAINER, POSTGRES_PASSWORD; compose.yaml, README.md, profiles.example.json updated.

Closes #3

Test plan

• gofmt -l ., go vet ./..., go build ./... — clean
• go test ./... — all packages green
• 8 unit tests (fields/address, safe-name, stream flags, -t args, awk FK filter, Permanent, stage markers)
• 6 integration tests on two postgres:17-alpine containers over a private network: preflight, whole-db clone (schemas, view, matview data, function, trigger, sequence setval, re-run clean), -only subset (dangling FK skipped+warned), -fresh under an open connection, remote→remote, auth fast-fail
• Coverage 90.3% on internal/driver/postgres