gallery: size model variants with a live probe, drop the nightly denormalizer

Selection needs each variant's size to decide whether it fits and to rank
largest-first. That figure was written into the index by a nightly job, which
made the gallery carry a derived value that could drift from the entry it was
derived from. Derive it at install time instead.

pkg/vram already sizes a model without downloading it, and the gallery UI
already uses it: a remote GGUF header range-fetch, then an HTTP HEAD for the
content length, then any declared size:. It caches its results, so reuse it
rather than writing a second probing path.

A probe failure must never fail an install, so an unprobeable variant is
treated as unknown: it survives the memory filter, because nothing proves it
does not fit, and it ranks last, so a known-good fit always beats a guess. If
every probe fails, selection still terminates on the base entry.

The probe is injected through ResolveEnv rather than called directly, for the
same reason the backend compatibility check is: specs pin an exact size, or an
exact failure, without reaching the network.

With that in place three things are dead weight and go:

- The nightly job and the fields it populated. Variant.Backend was redundant
  because the backend is resolved live from the referenced entry during
  selection, and Quantization was display-only that nothing read.
- min_memory on the base entry. The base always installs and its floor could
  only warn, so it could not change any outcome.
- The lint rules and schema entries for both.

min_memory on individual variants stays, as the override for when the probed
size is wrong. An authored figure now suppresses the probe entirely rather
than merely outranking it, so it costs no round trip.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

Author: mudler