ci: test on PostgreSQL 13 and document it as the minimum

.github/workflows/_checks.yml

@@ -97,6 +97,8 @@ jobs:
     # the same versions can still disagree on whether those versions install.
     # 3.14t is out: the dev group's `faststream[kafka]` pulls aiokafka, which ships no cp314t
     # wheel, so under --no-build this leg would measure upstream's gap rather than a floor.
+    # PostgreSQL 13 is the server floor: outbox_autovacuum_ddl sets the insert-triggered
+    # reloptions that 13 introduced.
     if: github.event_name != 'schedule'
     runs-on: ubuntu-latest
     strategy:
@@ -109,7 +111,7 @@ jobs:
           - "3.14"
     services:
       postgres:
-        image: postgres:17
+        image: postgres:13
         env:
           POSTGRES_USER: outbox
           POSTGRES_PASSWORD: outbox

docs/introduction/installation.md

@@ -23,10 +23,10 @@
 ## Requirements
 
 - Python 3.11+
-- PostgreSQL 12+. The features used (partial indexes, `FOR UPDATE SKIP
-  LOCKED`, `make_interval`, `pg_notify`) all predate 12. The examples and
-  CI run on 17; that is what's exercised, so 17 is the safest choice if
-  you're starting fresh.
+- PostgreSQL 13+. `outbox_autovacuum_ddl` sets the insert-triggered
+  autovacuum options that 13 introduced; on 12 the `ALTER TABLE` fails.
+  CI runs the test suite on 13 and on 17. The examples use 17, so that is
+  the safest choice if you're starting fresh.
 - A running Postgres instance accessible via SQLAlchemy `AsyncEngine`
 
 ## Free-threaded Python

docs/operations/alembic.md

@@ -209,7 +209,7 @@ def upgrade() -> None:
 ```
 
 This sets `autovacuum_vacuum_scale_factor = 0` and `autovacuum_vacuum_threshold =
-1000` (plus the insert-triggered pair, Postgres 13+). Tune the thresholds for your
+1000`, plus the insert-triggered pair, which needs PostgreSQL 13 or later. Tune the thresholds for your
 message rate, for example `outbox_autovacuum_ddl("outbox", vacuum_threshold=5000,
 insert_threshold=5000)`.