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/NestDatabase.cs

@@ -142,6 +142,9 @@ public sealed partial class NestDatabase : IDisposable
         lock (_sync)
         {
             using var transaction = _connection.BeginTransaction();
+            // Ids are client-allocated: an existing id, active or deleted, is a conflict, not a replacement.
+            if ((Get(id) ?? Get(id, deleted: true)) is { } existing)
+                throw new RevisionConflictException(existing, "A nest with this id already exists.");
             using var command = _connection.CreateCommand();
             command.Transaction = transaction;
             command.CommandText = """
@@ -488,7 +491,8 @@ public sealed partial class NestDatabase : IDisposable
     }
 }
 
-public sealed class RevisionConflictException(NestRecord current) : Exception("The nest has changed. Reload or Save As.")
+public sealed class RevisionConflictException(NestRecord current, string message = "The nest has changed. Reload or Save As.")
+    : Exception(message)
 {
     public NestRecord Current { get; } = current;
 }