Image models previously could not be scheduled through a priority
chain: the chat AUTO chain explicitly skips image-kind slots, and
AUTO image requests fell back to unordered registry discovery.
- config: add auto_image rules (auto_image yaml / image_rules json);
legacy auto rules keep their meaning as the chat chain
- core: buildAutoImageChain mirrors buildAutoChain with inverted kind
filter (image-only); SaveAutoImageRules + AutoImageRules/AutoImageChain
- scheduler: ChainImage walks the chain tier-by-tier with round-robin
and preference ordering, skipping cooling slots
- gateway: handleImage AUTO now runs down AutoImageChain when one is
configured (falls back to legacy discovery otherwise) and records
the actual served model; handleAutoAPI GET returns image_rules and
PUT accepts image_rules independently of rules
- webui: priority page gains a chat/image toggle editing two
independent lane sets; add-slot picker filters by active kind;
persistAuto writes only the active chain's field
