Type Safety: Add discriminated unions for result types

[thesubtleties]

Summary

Several result types use { success: boolean; error?: string } pattern which allows invalid states (success=true with error message).

Current State

• VodSyncResult in db/sync.ts
• Various hook return types

Proposed Solution

type SyncResult =
  | { success: true; movieCount: number; seriesCount: number; /* ... */ }
  | { success: false; error: string };

Benefits

• Invalid states become unrepresentable
• Better TypeScript inference
• Clearer error handling at call sites

From PR #6 Review

Type design synthesis rated overall type safety at 5.9/10, with discriminated unions as key improvement area.