Create global stats table

[jaeheonshim]

Database table to store cached global stats, and analogous JPA entity to represent the table.

Since this is the first issue, let's also start structuring how we want this feature to live inside the codebase. I think a new root-level package global_stats would be appropriate, with the new entity we create here living in the domain package within that package.

Table name: global_stats
Columns

• id
• created_at
• updated_at
• metric (all-lowercase slug identifying the metric)
• value (this can just be a bigint)

Acceptance Criteria

• Record new table in DBML schema
• Create flyway migration file for new table
• Flyway migration applies successfully locally
• Create JPA entity matching table definition in DB schema
• Hibernate successfully validates JPA entity against DB