feat: Convert condition_grade to dropdown with human-readable labels and descriptions

Summary

The condition_grade field is currently a free-form text input that expects specific enum values (FINE, VERY_GOOD, GOOD, FAIR, POOR). This causes validation errors when legacy data or user input doesn't match exactly.

Current Behavior

• Text input allows any value
• Backend expects uppercase enum values
• Legacy data may have "Good" instead of "GOOD"
• Cryptic validation errors shown to users

Proposed Solution

Convert to a dropdown with:

• Human-readable labels (e.g., "Very Good" displays, "VERY_GOOD" stored)
• Brief descriptions explaining each grade level, e.g.:
• Fine: Near perfect, minimal wear
• Very Good: Minor wear, tight binding, clean pages
• Good: Average used condition, some wear visible
• Fair: Heavy wear but complete and readable
• Poor: Significant damage or defects

Files to Modify

• frontend/src/components/books/BookForm.vue - Replace text input with select
• frontend/src/constants/index.ts - Add CONDITION_GRADE_OPTIONS with labels/descriptions
• Consider: Backend migration to normalize existing data

Related

• Also applies to provenance_tier which uses "Tier 1" instead of "TIER_1"