The dashboard's Config page was returning [unimplemented] because
trading-service had no ConfigService implementation. The API gateway's
ConfigServiceProxy forwards config RPCs to trading-service which now
handles them via direct SQL against the `configuration` table.
Implements GetConfiguration, UpdateConfiguration, DeleteConfiguration,
and ListCategories. Remaining 8 RPCs return UNIMPLEMENTED until needed.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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