feat: Add treplica helper for PostgreSQL streaming read replicas

[O1eq]

Description

Adds a new bin/treplica command that creates and manages a PostgreSQL streaming read replica of any totara pgsql container. Its purpose is testing Totara's database read/write splitting feature ($CFG->dboptions['readonly']), which routes read queries to a read-only replica.

• treplica up [pgsqlNN] - clones the running primary with pg_basebackup into a new volume and starts it as a hot standby (default service: pgsql16). Image, PGDATA and resource settings (max_connections, max_locks_per_transaction, ...) are derived from the running primary, so it works with any pgsql version/config without hardcoded values. Idempotent: re-running shows status, a stopped replica is restarted, a stale volume is refused with a hint. Ends by printing the ready-to-paste config.php block (including a warning that it must be placed after the config-after.phpinclude, which reassigns $CFG->dboptions).
• treplica status - recovery check, pg_stat_replication, replication lag.
• treplica logs - follows the standby's statement log.
• treplica down - removes container + volume after confirmation and drop the replication slot if one was created.
• Options: --port (publish on a host port), --slot (physical replication slot for bulk-load safety), --quiet-logs.

Testing Instructions

1. Check out this pull request
2. Have any Totara site running on a pgsql container (e.g. pgsql16)
3. Run treplica up - it should finish with state: streaming / Replica lag: in sync and print a config.php snippet
4. Run treplica status - same healthy output; run treplica up again - it should no-op politely
5. (Optional, needs a t21/TL-36188 site) paste the printed readonly block at the end of your config.php, set $CFG->perfdebug = 15;, reload any page and check the footer shows "DB reads from replica" counting up, while treplica logs shows only SELECT statements
6. Run treplica down, confirm - container, volume (and slot, if --slot was used) are removed; treplica status now reports there is no replica

Checklist

• Does what the author says it will do
• Testing instructions are provided
• Commit messages make sense and follow the conventional commit standard
• No identified security issues
• No identified maintenance issues
• Any third-party libraries/dependencies use the MIT or Apache 2.0 license
• Changes made are backwards compatible and will not break existing setups
• Changes to scripts in the bin/ directory run correctly on both MacOS and WSL
• Changes to containers can be built locally sucessfully (e.g. via tbuild container && tup container)
• Containers/images are compatible with both AMD64 (Windows) and ARM64 (MacOS)
• Changes made to config.php are compatible with our oldest supported Totara version, our newest Totara version, and Moodle