fix(BA-6899): stop app_configs downgrade from creating its enum twice

[jopemachine]

Problem

84d5c6daf8cc's downgrade() fails every time, on every database. Any downgrade that has to cross this revision is blocked.

It creates app_config_scope_type twice within itself:

app_config_scope_type = sa.Enum("DOMAIN", "PROJECT", "USER", name="app_config_scope_type")
app_config_scope_type.create(op.get_bind(), checkfirst=True)   # 1. creates it
op.create_table(
    "app_configs",
    ...
    sa.Column("scope_type", app_config_scope_type, ...),       # 2. emits CREATE TYPE again

upgrade() drops the type, so at this point it is absent and step 1 creates it. Step 2 then re-emits CREATE TYPE for the same enum with no existence check:

asyncpg.exceptions.DuplicateObjectError: type "app_config_scope_type" already exists
[SQL: CREATE TYPE app_config_scope_type AS ENUM ('DOMAIN', 'PROJECT', 'USER')]

This is not environment-specific and not an artifact of the one-shot bootstrap. On a clean DB built at head, with the enum confirmed absent (ABSENT at head) and no model declaring it, stamping 84d5c6daf8cc and running only its downgrade reproduces the failure deterministically.

Fix

Delete the explicit create() and let create_table() create the type once. upgrade() already does DROP TYPE IF EXISTS, so the pair round-trips.

Verification

Run against a real DB built at head, before and after:

step	before	after
downgrade 84d5c6daf8cc -> f3a8c1d05e64	DuplicateObjectError	exit 0 — app_configs restored, enum present, scope_type is USER-DEFINED/app_config_scope_type
upgrade back to 84d5c6daf8cc	—	exit 0 — table and enum dropped again
downgrade a second time	—	exit 0

Found while verifying the downgrade chain for #12881.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes the legacy app_configs migration downgrade by preventing duplicate PostgreSQL enum creation.

Changes:

• Lets op.create_table() create app_config_scope_type.
• Adds a changelog fragment for the downgrade fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
84d5c6daf8cc_drop_legacy_app_configs_table.py	Removes redundant enum creation.
changes/12887.fix.md	Documents the migration fix.

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[fregataa]

I don't understand how duplicate error happens. It seems upgrade should drop the app config scope enum type properly

[jopemachine]

I don't understand how duplicate error happens. It seems upgrade should drop the app config scope enum type properly

It's not that the enum wasn't removed during the upgrade. The issue is that the downgrade attempts to create the enum a second time. As a result, the downgrade fails even when the enum doesn't exist. @fregataa