PostgreSQL backend remaining features

[mostafa]

Summary

Finish the last PostgreSQL conversion gaps: temporal_ordered via LAG/LEAD, prepared output format, and value-modifier transforms.

Motivation

The Postgres backend is broadly shipped; a few constructs still degrade or error. Closing them improves hunt and convert fidelity for Timescale/Postgres archives.

Proposed approach

• Implement temporal_ordered windowing with LAG/LEAD.
• Add prepared output format.
• Cover remaining value-modifier transforms with goldens.

Out of scope

• New non-Postgres backends.
• Daemon-side SQL execution (hunt run is separate).

Tasks

• temporal_ordered SQL generation + goldens
• prepared format
• value-modifier transforms
• Docs updates

References

• Postgres backend module in rsigma-convert
• Existing backend golden suite