data ingest: image size is a CHECK + minimum-size floor (RFC-0002)

[LukasWodka]

RFC-0002 §12.9. Image size is a check, not a resize — and add a minimum-size floor.

• Reframe the --target-size flag + prompt as an expected size the images must already match (the ingestor validates exact equality, it never resizes). Keep --target-size as a hidden alias.
• Net-new: a minimum-image-size floor — there is none today; reject/​warn images below it. Needs the floor in the data-ingestors ImageResolutionValidator + the CLI preflight (ValidateImages), kept in parity via the golden harness. Decide the floor's source of truth (hardcoded vs a file_options.min_size schema field).

Design-of-record: RFC-0002 (#174). Part of the data-ingest redesign epic.

[LukasWodka]

Re-scoped per RFC-0002 Principle 6 + Option A (cli#174 §14): the minimum-size floor is new validation logic, so it lands in the ingestor first — data-ingestors#348. This ticket keeps the CLI-side pieces: the resize→check wording/flag rename, and the CLI preview of the min-size check, kept in lockstep with the ingestor by the value-level parity harness (backend#1009). Depends on data-ingestors#348.