fix(server): reject duplicate nest ids, invalid metadata and bare --database

- POST /api/nests with an existing id (active or deleted) now returns 409 with
  the current record instead of a 500 from the UNIQUE constraint.
- Uploaded nest metadata must have string text fields and a defined status;
  null fields and undefined statuses (e.g. 99) are 400 instead of a 500 or a
  stored invalid value.
- --database <path> is accepted alongside --database=<path>; a missing or
  repeated path fails startup instead of silently using OPENNEST_DB/default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Server/NestMetadata.cs

@@ -0,0 +1,60 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using OpenNest.Data;
+
+namespace OpenNest.Server;
+
+/// <summary>
+/// Reads and checks uploaded nest metadata. Text fields must be present and the status
+/// must be a defined value, so invalid metadata is a 400 and never reaches storage.
+/// </summary>
+internal static class NestMetadata
+{
+    // Integer statuses stay accepted for existing callers; GetError refuses undefined ones.
+    public static readonly JsonSerializerOptions Options = new(JsonSerializerDefaults.Web)
+    {
+        PropertyNameCaseInsensitive = true,
+        Converters = { new JsonStringEnumConverter(JsonNamingPolicy.CamelCase) },
+    };
+
+    public static bool TryRead(string? json, out NestRecord record, out string error)
+    {
+        record = new NestRecord();
+        if (string.IsNullOrWhiteSpace(json))
+        {
+            error = "Missing 'metadata' part.";
+            return false;
+        }
+
+        try
+        {
+            if (JsonSerializer.Deserialize<NestRecord>(json, Options) is not { } parsed)
+            {
+                error = "Missing metadata.";
+                return false;
+            }
+
+            record = parsed;
+        }
+        catch (JsonException)
+        {
+            error = "Invalid metadata JSON.";
+            return false;
+        }
+
+        error = GetError(record) ?? "";
+        return error.Length == 0;
+    }
+
+    public static string? GetError(NestRecord record)
+    {
+        if (record.Name is null) return "name must be a string.";
+        if (record.Customer is null) return "customer must be a string.";
+        if (record.Material is null) return "material must be a string.";
+        if (record.Comments is null) return "comments must be a string.";
+        if (record.MadeBy is null) return "madeBy must be a string.";
+        if (!Enum.IsDefined(record.Status)) return "status must be quote, toBeCut or hasBeenCut.";
+        if (!double.IsFinite(record.Thickness)) return "thickness must be a finite number.";
+        return null;
+    }
+}