Keep the feed catalogue and server settings in the database

Phase 3 of #18. Two tables: catalogue (each feed's config::Feed as JSON, so a
new feed setting needs no column) and settings (general: the five server
settings the admin page edits). config.toml keeps what is needed before the
database is reached, or decides who gets in: paths, [torrent], [web].

ipx still runs from one in-memory Config, assembled at start from both
(assemble_config). The eight places that saved config.toml and re-read it now
call Ctx::store_cfg, which writes the database and swaps the copy in memory; the
first-run web token, which is config.toml's, is written there.

The first start on a database with no catalogue imports config.toml's feeds and
settings in one transaction whose first insert is the settings row, so two ipx
starting at once cannot both import; it then trims config.toml, keeping the
original as config.toml.pre-database. After that, feeds written into the file are
ignored with a warning. copy-db skips it, and copies both tables.

Rehearsed on a clone of production's database with production's config: all 130
feeds imported, the file trimmed, and the feed list, settings and directory
identical to the live server's.

Postgres connections now ask for no notices. Every CREATE ... IF NOT EXISTS on an
existing table sends one, eleven per open; sqlx logs them, and
tracing-subscriber 0.3.23's per-layer filters then dropped the next line ipx
logged -- the import's own message went missing that way. Proved by toggling it.

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

CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - 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.
+- The catalogue of feeds and the server settings the admin page edits are kept in the database
+  rather than config.toml, which keeps where things are, the torrent settings and who may sign
+  in. The first start takes them from config.toml and trims it, keeping the original as
+  `config.toml.pre-database`; feeds added to config.toml after that are ignored, with a warning.
 
 ### Changed