refactor: migrate db and config packages to use struct-based API

Removed global variables in favor of instance variables. This makes the code more maintainable and making it easier to write unit tests without relying on global state.

Marked the old functions with global state as obsolete, redirecting them to use a new API based on struct in order to rewrite the code using these functions gradually.

internal/db/json.go

@@ -0,0 +1,64 @@
+package db
+
+import (
+	"database/sql"
+	"database/sql/driver"
+	"encoding/json"
+	"errors"
+	"fmt"
+)
+
+// JSON represents a JSON value in the database
+type JSON[T any] struct {
+	Val T
+}
+
+// NewJSON creates a new database JSON value
+func NewJSON[T any](v T) JSON[T] {
+	return JSON[T]{Val: v}
+}
+
+func (s *JSON[T]) Scan(val any) error {
+	if val == nil {
+		return nil
+	}
+
+	switch val := val.(type) {
+	case string:
+		err := json.Unmarshal([]byte(val), &s.Val)
+		if err != nil {
+			return err
+		}
+	case sql.NullString:
+		if val.Valid {
+			err := json.Unmarshal([]byte(val.String), &s.Val)
+			if err != nil {
+				return err
+			}
+		}
+	default:
+		return errors.New("sqlite json types must be strings")
+	}
+
+	return nil
+}
+
+func (s JSON[T]) Value() (driver.Value, error) {
+	data, err := json.Marshal(s.Val)
+	if err != nil {
+		return nil, err
+	}
+	return string(data), nil
+}
+
+func (s JSON[T]) MarshalYAML() (any, error) {
+	return s.Val, nil
+}
+
+func (s JSON[T]) String() string {
+	return fmt.Sprint(s.Val)
+}
+
+func (s JSON[T]) GoString() string {
+	return fmt.Sprintf("%#v", s.Val)
+}