refactor(providers): a provider is a vendor and a key, and the table is gone

The dropdown was listing "connections" — a row per adapter + model + key — which
made choosing a model an act of configuration, put a key icon and a bin inside a
menu item, and showed five accounts nobody had created.

A provider is now a vendor Officer curates. Every one is listed, set up or not,
because a vendor you have no key for is exactly what you need to see in order to
add one. The model is chosen per generation, from that provider's entry in the
catalogue, beside it.

With the model off the row, all that remained of a "provider" was the key for
OpenAI — and Officer already stores keys: one active value per purpose, 0600
inside a 0700 directory, outside the backup tarball, created on first use so
nothing needs provisioning. So `atelier_providers` is dropped, along with
`generations.provider_id` that pointed into it. `provider` and `model` stay as
text on each generation, which were always the historical fact rather than a
pointer to configuration.

`getSecret`, never `getKey`: this is a foreign credential, and getKey mints a
random value for a purpose it cannot find — which for someone else's API key
means authenticating with something invented.

Resolution follows: a request names a model, and the catalogue says whose it is,
rather than a database being asked which stored row served it. A model nobody
in the catalogue claims is still accepted — the catalogue is advisory, and a
model released today should work before we ship a file that mentions it.

Two routes replace five: PUT and DELETE on /providers/:name/key. Deleting a key
that came from the environment is refused rather than faked — it belongs to the
install, not to this screen.

sidecar/db/index.ts

@@ -7,7 +7,6 @@
 // stay in the `atelier` database, untouched. See PLUGIN.md.
 export * from './generations';
 export * from './collections';
-export * from './providers';
 
 // The connection itself, so callers do not reach past the package for it.
 export { db, waitForDatabase } from './db';