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.Tests/ServerOptionsTests.cs

@@ -0,0 +1,34 @@
+namespace OpenNest.Server.Tests;
+
+public sealed class ServerOptionsTests
+{
+    private static readonly string Default = Path.Combine("data", "nests.db");
+
+    [Theory]
+    [InlineData(new[] { "--database=/srv/a.db" }, "/srv/a.db")]
+    [InlineData(new[] { "--database", "/srv/a.db" }, "/srv/a.db")]
+    [InlineData(new[] { "--urls=http://127.0.0.1:8090", "--database", "/srv/a.db" }, "/srv/a.db")]
+    [InlineData(new[] { "--database=/srv/a=b.db" }, "/srv/a=b.db")]
+    public void DatabasePath_AcceptsBothForms_AndOverridesEnvironment(string[] args, string expected)
+    {
+        Assert.Equal(expected, ServerOptions.DatabasePath(args, "/env/nests.db"));
+    }
+
+    [Fact]
+    public void DatabasePath_WithoutArgument_UsesEnvironmentThenDefault()
+    {
+        Assert.Equal("/env/nests.db", ServerOptions.DatabasePath(["--urls=http://127.0.0.1:8090"], "/env/nests.db"));
+        Assert.Equal(Default, ServerOptions.DatabasePath([], null));
+    }
+
+    [Theory]
+    [InlineData("--database")]
+    [InlineData("--database", "--urls=http://127.0.0.1:8090")]
+    [InlineData("--database=")]
+    [InlineData("--database=  ")]
+    [InlineData("--database=/a.db", "--database", "/b.db")]
+    public void DatabasePath_MissingOrRepeated_FailsInsteadOfFallingBack(params string[] args)
+    {
+        Assert.Throws<ArgumentException>(() => ServerOptions.DatabasePath(args, "/env/nests.db"));
+    }
+}