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/ServerOptions.cs

@@ -0,0 +1,43 @@
+namespace OpenNest.Server;
+
+/// <summary>Startup settings read from the command line and environment.</summary>
+public static class ServerOptions
+{
+    public const string DatabaseVariable = "OPENNEST_DB";
+
+    /// <summary>
+    /// The database path: <c>--database=&lt;path&gt;</c> or <c>--database &lt;path&gt;</c>, then
+    /// <c>OPENNEST_DB</c>, then <c>./data/nests.db</c>. A <c>--database</c> without a path,
+    /// or given twice, fails startup instead of silently storing nests elsewhere.
+    /// </summary>
+    public static string DatabasePath(IReadOnlyList<string> args, string? environmentPath)
+    {
+        string? path = null;
+        for (var i = 0; i < args.Count; i++)
+        {
+            string value;
+            if (args[i] == "--database")
+            {
+                if (i + 1 >= args.Count || args[i + 1].StartsWith("--", StringComparison.Ordinal))
+                    throw new ArgumentException("--database requires a path.", nameof(args));
+                value = args[++i];
+            }
+            else if (args[i].StartsWith("--database=", StringComparison.Ordinal))
+            {
+                value = args[i]["--database=".Length..];
+            }
+            else
+            {
+                continue;
+            }
+
+            if (string.IsNullOrWhiteSpace(value))
+                throw new ArgumentException("--database requires a path.", nameof(args));
+            if (path is not null)
+                throw new ArgumentException("--database was given more than once.", nameof(args));
+            path = value;
+        }
+
+        return path ?? environmentPath ?? Path.Combine("data", "nests.db");
+    }
+}