Refactor: Replace @ManyToMany with intermediary entity and improve Entity stability

[gratisreise]

1. Key Changes

• Introduction of Intermediary Entity: Replaced @manytomany with a formal ProductTag entity, refactoring the relationship into @onetomany and @manytoone associations.
• Enhanced Entity Stability: Removed the risky @DaTa annotation in favor of @Getter and @NoArgsConstructor(access = AccessLevel.PROTECTED) to ensure encapsulation and identifier integrity.
• Explicit ID Generation: Replaced ambiguous @GeneratedValue with GenerationType.IDENTITY for better database predictability.
• Optimized Fetching: Updated the NamedEntityGraph with Subgraphs to ensure Product, ProductTag, and Tag can still be fetched in a single optimized query.

2. Expected Benefits

• Extensibility: Allows for adding relationship-specific metadata (e.g., created_at, display_order) to the ProductTag entity without schema overhauls.
• Runtime Reliability: Eliminates potential StackOverflowError during serialization and prevents HashSet inconsistency caused by mutable hashCode values in entities.
• Query Control: Provides explicit control over the intermediary table, preventing performance degradation (such as the "Hibernate Bag" issue) during bulk data operations.

[gratisreise]

here is my PR #710

[schauder]

all the things you try to improve here aren't really relevant for an example for a special feature.