Add recommendation provenance and retrieval timestamps

[unes07]

Problem

Recommendation rows expose agronomic inputs and outputs without recording when the upstream response was retrieved or which endpoint produced it. Generic-formula costs also lack upstream price date, supplier, and methodology metadata.

Proposed work

• Add a timezone-aware UTC retrieval timestamp to every recommendation row.
• Record the upstream source endpoint or stable source identifier.
• Expose any upstream provenance metadata when available.
• Represent unavailable price date, supplier, and methodology as explicit null values rather than inferred values.
• Preserve the fields through single-site, batch, Python, JSON, and CSV workflows.

Acceptance criteria

• Retrieval timestamps are timezone-aware and serialized consistently.
• Single-site and batch rows include the same provenance schema.
• Unknown metadata remains null and is not guessed.
• Tests cover Python, JSON, CSV, and batch outputs.
• Documentation distinguishes retrieval time from an upstream price-observation time.

Related to #2, #3, and #4.