v0.1.0-rc9 Added typed task Priority API

✨ Features
- tasks: Introduce typed Priority API for enhanced type safety.
  Replaces *int with *Priority, requiring method signature changes
  (e.g., WithPriority(int) to Priority(Priority) on task builders).
  Includes new Priority constants, String()/Valid() methods,
  ParsePriority(), JSON marshal/unmarshal support, and sentinel errors
  (ErrInvalidPriority, ErrPriorityOutOfRange).
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