Plan the artist platform: price calculator and fan-funded pressing

New docs/artist-platform.md: upload master and artwork with automatic
audio, artwork and rights checks; an itemised price calculator for every
customer (per-title vs per-copy lines, price-per-copy curve, lead time
from the production-timeline quote table, versioned price list); and
fan-funded campaigns that press and dropship per fan once the goal is
reached, refund otherwise. Covers goal maths, payments (charge into a
ring-fenced account, CNX as seller of record, legal opinion first; the
Qrates failure as the design rule), repress pre-orders, architecture on
the cnx-factory stack with the public side on a fleet machine, PDPA,
phasing (calculator public early as a run-size probe, campaigns only
after first releases ship), cost and open decisions.

Backlinks from README, process flow, production timeline, revenue model,
digital factory and roadmap items 1 and 20; total investment added-scope
table gains the platform line.

docs/digital-factory.md

@@ -508,7 +508,11 @@ Modules, in build order:
    against the BOM, carrier and tracking per shipment or dropship end
    customer.
 10. **Customer portal**: per-order timeline of everything above, scoped so
-    one customer sees only its own orders; PDF export of the trace.
+    one customer sees only its own orders; PDF export of the trace. The
+    public front of order intake (price calculator, uploads with
+    automatic checks, fan-funded campaigns) is planned in
+    [artist platform](artist-platform.md), including why its public
+    side should not depend on the plant's WAN link.
 11. **Reports and signals**: yield per stamper, reject rate drift, audio
     score trend versus play count (the automatic retirement signal),
     energy per record, environment excursions overlaid on runs (the