feat: replace VocabularyScope enum with Path-based scope hierarchy

[mdproctor]

Background

Evaluation in #190 concluded that VocabularyScope (GLOBAL/ORG/TEAM/PERSONAL enum) should be replaced with Path-based scope using isAncestorOf() instead of ordinal comparison. SettingsScope from casehub-platform-api is not the right target — it carries Instant effectiveAt semantics irrelevant here — but Path directly is the right type.

What needs to change

• LabelVocabulary.scope: VocabularyScope enum → Path (with PathAttributeConverter)
• isScopeAccessible(VocabularyScope, VocabularyScope) → isAccessibleFrom(Path callerPath) using Path.isAncestorOf()
• REST API: scope path param changes from GLOBAL/ORG/TEAM/PERSONAL strings to explicit path strings
• Flyway migration: convert existing enum values (GLOBAL → root, ORG/TEAM/PERSONAL → segment-based paths)
• Delete VocabularyScope enum

Scale: M · Med