This is a long pending refactor. All the DB, query, CRUD, and related
logic scattered across HTTP handlers are now moved into a central
`core` package with clean, abstracted methods, decoupling HTTP
handlers from executing direct DB queries and other business logic.
eg: `core.CreateList()`, `core.GetLists()` etc.
- Remove obsolete subscriber methods.
- Move optin hook queries to core.
- Move campaign methods to `core`.
- Move all campaign methods to `core`.
- Move public page functions to `core`.
- Move all template functions to `core`.
- Move media and settings function to `core`.
- Move handler middleware functions to `core`.
- Move all bounce functions to `core`.
- Move all dashboard functions to `core`.
- Fix GetLists() not honouring type
- Fix unwrapped JSON responses.
- Clean up obsolete pre-core util function.
- Replace SQL array null check with cardinality check.
- Fix missing validations in `core` queries.
- Remove superfluous deps on internal `subimporter`.
- Add dashboard functions to `core`.
- Fix broken domain ban check.
- Fix broken subscriber check middleware.
- Remove redundant error handling.
- Remove obsolete functions.
- Remove obsolete structs.
- Remove obsolete queries and DB functions.
- Document the `core` package.
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