Wire custom providers end-to-end (admin API, boot default, model picker) (#512)

The custom-provider subsystem (store, admin routes, runtime registry, catalog) existed but was unreachable in default production wiring. Three fixes: the server-deps assembly never passed the custom-provider store and refresher, so the admin API always 404'd — the assembly now lives next to buildApp so it cannot drift, and boot awaits the initial refresh; the boot default resolved the configured model eagerly before custom providers hydrated, so a configured custom model silently degraded to the built-in default forever — the configured model id is now passed raw and resolved lazily per call; and the web-ui picker only accepted dynamic catalog entries for the openrouter provider and threw for custom-provider slugs — any catalog-backed model now clones the dynamic template while keeping its own provider slug. An end-to-end boot-wiring test exercises production-shaped wiring from registration through the runtime-config advertisement.  marcclefas-code

**Deployment notes**

Release-note the flip: configured custom models take effect after upgrade; orgs should verify
their custom-provider config is actually valid
Deliberate behavior flip (fixes public #269/#282/#333): a configured custom default model that
previously degraded silently to the built-in default at boot is now resolved lazily and actually used
— an org with stale/broken custom-provider config will see per-call failures where it silently got
the default before
Admin custom-providers API goes from always-404 to live (routes existed, deps were never
wired); endpoints sit behind the existing admin auth
index.ts now awaits refreshCustomProviders() at boot — one extra store read at startup, same
database; no schema change

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