fix(app-space): expand GROUP path items + use per-path description

[jung-thomas]

Problem

Two rendering bugs on the App-Space event page (e.g. /app-space/?eventId=10000015), reported against the "SAP TechEd Berlin 2026 Test" event:

1. A track whose path item is a GROUP renders "0 tutorials". The "Joule Studio (web)" track contains a single GROUP path item (its tutorials live in the group), but the card shows 0 tutorials.
2. Every track shows the same description — the mission's "Ready to get your hands dirty?…" text — instead of each Completion Path's own description.

Root cause

Both live in getAppSpaceProgress (and its twin getEventProgress) in srv/developer-service.js:

1. The handler emitted one output item per CompletionPathItems row, passing type: i.taskType through verbatim. A GROUP row became a single type:'GROUP' card, which the frontend island (AppSpace.vue, counts type === 'TUTORIAL') filters out → 0 tutorials. The group's child tutorials live in GroupPathItems and were never read. (build-catalog.js already expands nested groups — this ports that behavior.)
2. description: mission.description was hard-coded for every path, ignoring the CompletionPaths.description column that was already fetched.

Fix

• GROUP expansion: new expandGroupTutorials() in srv/lib/path-item-task-resolver.js batch-reads GroupPathItems (ordered by itemOrder) → child tutorials; buildPathItems() flattens each GROUP into one TUTORIAL output item per child. Child TaskRecord lookups are batched via groupChildLegacyIds() so progress/status still render. No frontend change needed — children arrive as TUTORIAL items.
• Per-path description: description: p.description || mission.description || ''.
• Shared paths-building extracted so both twin handlers stay in sync.

Tests

• New regression tests in test/app-space-fk-resolution.test.js: (a) GROUP path item expands into its ordered child tutorials with no leftover GROUP card; (b) each path shows its own description, distinct from the mission text. Both verified to fail against the old code and pass with the fix.
• npm test CAP backend suite green (developer-service 25/25, app-space 4/4). Pre-existing fresh-worktree artifact failures (*.bundle.mjs, @mediapipe/tasks-vision) are unrelated to this change.

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