Docs: Postgres in production

Where the database now is and how to reach it, IPX_DATABASE_URL and
IPX_TEST_DATABASE_URL, copy-db, backups, and the title sort on Postgres.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -7,10 +7,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- ipx can keep its data in Postgres: set `IPX_DATABASE_URL` to a `postgres://` URL. Without it,
+  it is the SQLite `state.db` as before. `ipx copy-db <state.db>` moves an existing database
+  across, everything in one go.
+
 ### Changed
 
 - The database is reached through SeaORM, on the way to Postgres (issue #18); it is still the
-  same SQLite file, and nothing you see changes. A database from before 0.7 has to be opened by a
+  same SQLite file, and nothing you see changes. On Postgres, sorting by title or feed follows
+  the language's order (an accented letter beside the plain one) rather than raw bytes. A database from before 0.7 has to be opened by a
   0.7 release first, which brings its tables up to date.
 - A pinned item sits at the top of its list, above everything else in whatever order you sort
   by, and moves there the moment you pin it. Sorting by the pin column itself still goes both