Bulk catalogue: model-year ranges and a leaner payload

Added

• years on every entry — { from, to, source }, the range of search years an entry should be used for. source is successor (a newer version exists), estimated (end estimated) or open (current generation, to is null).
• next_model_year — the next model year of the same vehicle, or null.
• colors_available — whether the car can be repainted via ?color=.

Improved

• One entry per vehicle instead of one per colour: the response is far smaller and faster. A car's colours are in its /colors endpoint.
• Within one model, variant and trim the year ranges never overlap — a given search year matches at most one entry.
• Dropped the constant format and resolution fields; both were always png / default.