feat: graphical model management in admin panel

Replace the provider credential fields and raw AI_MODELS_CONFIG JSON
textarea with a Models section mirroring the in-app model settings UI:
provider instance list with logos, credential fields per provider type,
model add/remove with suggestions, per-model connectivity test, and a
default-provider star.

On save the server derives everything the runtime needs into
settings.json: credential env vars (with _2 suffixes for multiple
instances of one provider), AI_MODELS_CONFIG, and AI_PROVIDER/AI_MODEL
for the default. Secrets round-trip as masked markers and are never
sent back to the browser. The general settings registry now only
covers non-provider settings (generation, access, features,
observability, quota).

app/api/admin/providers/route.ts

@@ -0,0 +1,71 @@
+import { checkAdminAuth } from "@/lib/admin/auth"
+import {
+    AdminProvidersSchema,
+    deriveEnvUpdates,
+    loadAdminProviders,
+    maskAdminProviders,
+    mergeSecrets,
+    validateAdminProviders,
+} from "@/lib/admin/providers"
+import { isSettingsWritable, saveSettings } from "@/lib/admin/settings"
+
+export const runtime = "nodejs"
+export const dynamic = "force-dynamic"
+
+function payload() {
+    return {
+        writable: isSettingsWritable(),
+        providers: maskAdminProviders(loadAdminProviders()),
+    }
+}
+
+export async function GET(req: Request) {
+    const authError = checkAdminAuth(req)
+    if (authError) return authError
+    return Response.json(payload())
+}
+
+export async function PUT(req: Request) {
+    const authError = checkAdminAuth(req)
+    if (authError) return authError
+
+    if (!isSettingsWritable()) {
+        return Response.json(
+            {
+                error: "Settings file is not writable on this deployment. Configure via environment variables instead.",
+            },
+            { status: 503 },
+        )
+    }
+
+    let body: unknown
+    try {
+        body = await req.json()
+    } catch {
+        return Response.json({ error: "Invalid JSON body" }, { status: 400 })
+    }
+
+    const parsed = AdminProvidersSchema.safeParse(
+        (body as { providers?: unknown })?.providers,
+    )
+    if (!parsed.success) {
+        return Response.json(
+            {
+                error: `Invalid providers: ${parsed.error.issues[0]?.message ?? "schema mismatch"}`,
+            },
+            { status: 400 },
+        )
+    }
+
+    const stored = loadAdminProviders()
+    const merged = mergeSecrets(parsed.data, stored)
+
+    const validationError = validateAdminProviders(merged)
+    if (validationError) {
+        return Response.json({ error: validationError }, { status: 400 })
+    }
+
+    saveSettings(deriveEnvUpdates(merged, stored))
+
+    return Response.json(payload())
+}