remove broadcast from template_type

we can't _just_ do the same process as 0467 because there are check
constraints that depend on these. When you say "alter column
template_type set type = new_type using old_type::text::new_type", it
takes that "using" statement as a way of translating the prev enum into
the new enum, in our case just casting to a str as an intermediate step.

however, the template_type column has several check constraints against
it (to say only letters can have a letter_attachment for example).
Postgres isn't smart enough to take the `using` clause and apply it to
these constraints. There are two ways around this:

* You could create a comparator function that tells postgres "if you
  have to compare old_type with new_type, here's a custom function"[^1]
* You can drop the old constraints and recreate them after the migration
  is done

We've gone with the second option as it's a bit simpler and I could copy
and paste from our previous scripts

[^1]: https://stackoverflow.com/a/57952345 was very helpful for this

migrations/versions/0468_remove_broadcast_type.py

@@ -0,0 +1,133 @@
+"""
+Create Date: 2024-10-01 11:08:46.900469
+"""
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+import textwrap
+
+revision = "0468_remove_broadcast_type"
+down_revision = "0467_remove_broadcast_perms"
+
+
+template_type_new_values = ["email", "sms", "letter"]
+template_type_new = postgresql.ENUM(*template_type_new_values, name="template_type")
+
+
+def drop_check_constraints():
+    op.drop_constraint("chk_templates_letter_languages", "templates")
+    op.drop_constraint("chk_templates_history_letter_languages", "templates_history")
+
+    op.drop_constraint("ck_templates_letter_attachments", "templates")
+    op.drop_constraint("ck_templates_history_letter_attachments", "templates_history")
+
+    op.drop_constraint("ck_templates_non_email_has_unsubscribe_false", "templates")
+    op.drop_constraint("ck_templates_history_non_email_has_unsubscribe_false", "templates_history")
+
+
+def add_check_constraints():
+    op.create_check_constraint(
+        "chk_templates_letter_languages",
+        "templates",
+        """
+        CASE WHEN template_type = 'letter' THEN
+            letter_languages is not null
+        ELSE
+            letter_languages is null
+        END
+        """,
+        postgresql_not_valid=True,
+    )
+    op.create_check_constraint(
+        "chk_templates_history_letter_languages",
+        "templates_history",
+        """
+        CASE WHEN template_type = 'letter' THEN
+            letter_languages is not null
+        ELSE
+            letter_languages is null
+        END
+        """,
+        postgresql_not_valid=True,
+    )
+
+    op.create_check_constraint(
+        "ck_templates_letter_attachments",
+        "templates",
+        "template_type = 'letter' OR letter_attachment_id IS NULL",
+        postgresql_not_valid=True,
+    )
+    op.create_check_constraint(
+        "ck_templates_history_letter_attachments",
+        "templates_history",
+        "template_type = 'letter' OR letter_attachment_id IS NULL",
+        postgresql_not_valid=True,
+    )
+
+    op.create_check_constraint(
+        "ck_templates_non_email_has_unsubscribe_false",
+        "templates",
+        "template_type = 'email' OR has_unsubscribe_link IS false",
+        postgresql_not_valid=True,
+    )
+    op.create_check_constraint(
+        "ck_templates_history_non_email_has_unsubscribe_false",
+        "templates_history",
+        "template_type = 'email' OR has_unsubscribe_link IS false",
+        postgresql_not_valid=True,
+    )
+
+
+def upgrade():
+    """
+    there are three check constraints on the template_type column (across both templates and templates_history).
+
+    These check constraints say things like "if type == 'letter'" - this is an enum comparison, so if we change the
+    type of the column, we get an error
+
+    > (psycopg2.errors.UndefinedFunction) operator does not exist: template_type = template_type_old
+    > HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.
+
+    This appears confusing because we're passing in a `using` clause to tell postgres exactly how to do this, but
+    this using clause only applies to the column type, and not other usages of that column within the check constraints.
+
+    To get round this, we "simply" drop all the constraints and then recreate them afterwards, referring to the new
+    """
+    conn = op.get_bind()
+    conn.execute("ALTER TYPE template_type RENAME TO template_type_old;")
+    template_type_new.create(conn)
+
+    drop_check_constraints()
+
+    op.alter_column(
+        table_name="templates",
+        column_name="template_type",
+        type_=template_type_new,
+        postgresql_using="template_type::text::template_type",
+    )
+    op.alter_column(
+        table_name="templates_history",
+        column_name="template_type",
+        type_=template_type_new,
+        postgresql_using="template_type::text::template_type",
+    )
+    op.alter_column(
+        table_name="service_contact_list",
+        column_name="template_type",
+        type_=template_type_new,
+        postgresql_using="template_type::text::template_type",
+    )
+
+    conn.execute("DROP TYPE template_type_old;")
+    # note: need to revalidate these constraints in a separate migration to avoid a lengthy access exclusive lock
+    add_check_constraints()
+
+
+def downgrade():
+    # don't need to do the constraints dance, or the enum type dance, when adding a new value
+
+    # ALTER TYPE must be run outside of a transaction block (see link below for details)
+    # https://alembic.sqlalchemy.org/en/latest/api/runtime.html#alembic.runtime.migration.MigrationContext.autocommit_block
+    with op.get_context().autocommit_block():
+        op.execute("ALTER TYPE template_type ADD VALUE 'broadcast'")

migrations/versions/0469_validate_template_ck.py

@@ -0,0 +1,26 @@
+"""
+Create Date: 2024-10-01 11:08:46.900469
+"""
+
+revision = "0469_validate_template_ck"
+down_revision = "0468_remove_broadcast_type"
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+
+def upgrade():
+
+    op.execute("ALTER TABLE templates VALIDATE CONSTRAINT chk_templates_letter_languages")
+    op.execute("ALTER TABLE templates_history VALIDATE CONSTRAINT chk_templates_history_letter_languages")
+
+    op.execute("ALTER TABLE templates VALIDATE CONSTRAINT ck_templates_letter_attachments")
+    op.execute("ALTER TABLE templates_history VALIDATE CONSTRAINT ck_templates_history_letter_attachments")
+
+    op.execute("ALTER TABLE templates VALIDATE CONSTRAINT ck_templates_non_email_has_unsubscribe_false")
+    op.execute("ALTER TABLE templates_history VALIDATE CONSTRAINT ck_templates_history_non_email_has_unsubscribe_false")
+
+
+def downgrade():
+    pass