feat: add file-based admin settings panel at /admin

Settings saved in the panel are written to data/settings.json and
overlaid onto process.env, taking precedence over environment
variables and applying immediately without restart. Enable by setting
ADMIN_PASSWORD; on serverless platforms without persistent disk the
panel degrades to read-only.

app/api/admin/settings/route.ts

@@ -0,0 +1,171 @@
+import { timingSafeEqual } from "crypto"
+import {
+    getEnvFallback,
+    getValueSource,
+    isSettingsWritable,
+    loadSettings,
+    saveSettings,
+} from "@/lib/admin/settings"
+import { SETTINGS_BY_KEY, type SettingDef } from "@/lib/admin/settings-registry"
+import { ServerModelsConfigSchema } from "@/lib/server-model-config"
+
+export const runtime = "nodejs"
+export const dynamic = "force-dynamic"
+
+function checkAuth(req: Request): Response | null {
+    const password = process.env.ADMIN_PASSWORD
+    if (!password) {
+        return Response.json(
+            {
+                error: "Admin panel is disabled. Set the ADMIN_PASSWORD environment variable to enable it.",
+            },
+            { status: 403 },
+        )
+    }
+    const provided = req.headers.get("x-admin-password") || ""
+    const a = Buffer.from(provided)
+    const b = Buffer.from(password)
+    if (a.length !== b.length || !timingSafeEqual(a, b)) {
+        return Response.json(
+            { error: "Invalid admin password" },
+            { status: 401 },
+        )
+    }
+    return null
+}
+
+function maskSecret(value: string): { isSet: true; hint: string } {
+    return {
+        isSet: true,
+        hint: value.length > 8 ? `…${value.slice(-4)}` : "••••",
+    }
+}
+
+function serializeSettings() {
+    const fileValues = loadSettings()
+    return [...SETTINGS_BY_KEY.values()].map((def) => {
+        const source = getValueSource(def.key)
+        const raw =
+            source === "file"
+                ? fileValues[def.key]
+                : (getEnvFallback(def.key) ?? null)
+
+        let value: unknown = raw
+        if (def.type === "secret" && raw) {
+            value = maskSecret(raw)
+        }
+        return { key: def.key, source, value }
+    })
+}
+
+export async function GET(req: Request) {
+    const authError = checkAuth(req)
+    if (authError) return authError
+
+    return Response.json({
+        writable: isSettingsWritable(),
+        settings: serializeSettings(),
+    })
+}
+
+function validateValue(def: SettingDef, value: string): string | null {
+    switch (def.type) {
+        case "number": {
+            const num = Number(value)
+            if (Number.isNaN(num)) return "Must be a number"
+            if (def.min !== undefined && num < def.min)
+                return `Must be at least ${def.min}`
+            if (def.max !== undefined && num > def.max)
+                return `Must be at most ${def.max}`
+            return null
+        }
+        case "boolean":
+            return value === "true" || value === "false"
+                ? null
+                : 'Must be "true" or "false"'
+        case "enum":
+            return def.options?.includes(value)
+                ? null
+                : `Must be one of: ${def.options?.join(", ")}`
+        case "json": {
+            let parsed: unknown
+            try {
+                parsed = JSON.parse(value)
+            } catch {
+                return "Invalid JSON"
+            }
+            if (def.key === "AI_MODELS_CONFIG") {
+                const result = ServerModelsConfigSchema.safeParse(parsed)
+                if (!result.success) {
+                    return `Invalid model registry: ${result.error.issues[0]?.message ?? "schema mismatch"}`
+                }
+            }
+            return null
+        }
+        default:
+            return null
+    }
+}
+
+export async function PUT(req: Request) {
+    const authError = checkAuth(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: { values?: Record<string, unknown> }
+    try {
+        body = await req.json()
+    } catch {
+        return Response.json({ error: "Invalid JSON body" }, { status: 400 })
+    }
+    if (!body.values || typeof body.values !== "object") {
+        return Response.json(
+            { error: "Body must contain a values object" },
+            { status: 400 },
+        )
+    }
+
+    const updates: Record<string, string | null> = {}
+    const errors: Record<string, string> = {}
+
+    for (const [key, value] of Object.entries(body.values)) {
+        const def = SETTINGS_BY_KEY.get(key)
+        if (!def) {
+            errors[key] = "Unknown setting"
+            continue
+        }
+        if (value === null || value === "") {
+            updates[key] = null
+            continue
+        }
+        if (typeof value !== "string") {
+            errors[key] = "Value must be a string"
+            continue
+        }
+        const error = validateValue(def, value)
+        if (error) {
+            errors[key] = error
+            continue
+        }
+        updates[key] = value
+    }
+
+    if (Object.keys(errors).length > 0) {
+        return Response.json({ errors }, { status: 400 })
+    }
+
+    saveSettings(updates)
+
+    return Response.json({
+        writable: true,
+        settings: serializeSettings(),
+    })
+}