refactor(api): ScreensService.add complexity and duplicated screen lookup

Problem Statement

packages/api/src/screens/screens.service.ts:

• add (line 34): cyclomatic 16, cognitive 18, 71 lines — high (fallow health)
• Clone group: delete (116-121) and updateExternalScreen (180-185) both do "find screen with device relation, warn + NotFoundException('Screen not found')".

Solution

• Extract private async findScreenWithDevice(id): Promise<Screen> that throws the NotFoundException, and use it from delete, updateExternalScreen and any other method doing the same lookup.
• Split add into the branches it handles (upload vs external link vs plugin/mashup source) as private methods so the public method is a dispatcher.
• screens/__test__ specs stay green.

Verification

• pnpm lint && pnpm type-check && pnpm test pass
• Run pnpm fallow:baseline in the same PR so the finding disappears from .fallow-baselines/, then confirm pnpm fallow:ci passes. Never add new findings to a baseline; see docs/agents/fallow.md.

Found by the initial fallow scan (pnpm fallow) on 2026-08-22 and baselined so CI stays green until this is fixed.