[TASK-SIM-004] feat: add visit date conflict validation

[BenjaminWong1205]

[TASK-SIM-004] feat: add visit date conflict validation

Summary

Prevent duplicate visit bookings for the same pet on the same date.

Changes

• ✅ Added @UniqueConstraint(columnNames = {"pet_id", "visit_date"}) to Visit entity
• ✅ Added in-memory conflict check in VisitController.processNewVisitForm
• ✅ Updated H2 / MySQL / PostgreSQL schema scripts with unique index
• ✅ Added 2 test cases: duplicate date (rejected) & different date (accepted)

Test Results

• ✅ All tests passed
• ✅ Coverage: VisitController ≥85%
• ✅ Spring Java Format check passed

Validation Layers

Layer	Mechanism	Fallback
Database	Unique index on (pet_id, visit_date)	Prevents concurrent inserts
Java	result.rejectValue() + in-memory check	User-friendly error message
Frontend	Thymeleaf field error rendering	Highlights duplicate date

Checklist

• Code compiles without errors
• All unit tests pass
• Code coverage ≥ 85%
• Multi-database schema updated (H2/MySQL/PostgreSQL)
• Code follows project conventions