Connectors: allow secrets to be entered in the UI, encrypted at rest

Reference-only secrets (SDD §6.4) meant adding a connector required editing a
file on the server and restarting the service. In practice that leads to the
token being pasted into the env-var *name* field, which fails with "environment
variable '<token>' is not set" and gives no hint what went wrong.

Add a second storage form, chosen per connector: type the secret in and it is
encrypted via ASP.NET Core data protection before it is stored. The env-var
reference stays as an equal alternative — this widens the choice rather than
replacing it. Exactly one form survives a save, so a stale secret cannot linger
and silently win; EndpointSecret.Resolve is the single resolution path.

The guarantee that matters is preserved: no plaintext in the database, so
pg_dump and JSON exports carry nothing usable. The trust boundary is stated
plainly in §6.4 — the key ring is on disk, so this protects against leaked
database content, not an attacker who already has the host, which is the same
boundary an env var has.

Details worth noting:
- Key ring defaults to /var/lib/metervault/keys, outside the app directory,
  because the LXC updater republishes /opt/metervault on every update. Docker
  gets a named volume. Overridable via MeterVault__DataProtectionKeyPath.
- Undecryptable ciphertext (key ring lost) falls back rather than throwing: an
  ingestion worker on a timer should degrade, not crash.
- The stored secret is never sent to the browser; a blank field means
  "unchanged", not "cleared".
- MQTT usernames are stored as-is — §6.4 covers tokens and passwords, and
  encrypting a username would only blank the field on every edit.
- ExportService drops *_enc values: bound to the originating key ring, so
  useless where an export would be restored. Expect to re-enter after a restore.
- HaConnectionTester now takes a resolved token, so the admin UI can test a
  token that has been typed but not yet saved.

SDD §6.4 and §9 updated to describe both forms rather than contradict the code.

Claude-Session: https://claude.ai/code/session_01V6joyergfvVLFEizH1hJLd

src/Infrastructure/Ingestion/HaConnectionTester.cs

@@ -7,32 +7,31 @@ namespace MeterVault.Infrastructure.Ingestion;
 public sealed record HaTestResult(bool Ok, string Message, double? SampleValue = null);
 
 /// <summary>
-/// Verifies a Home Assistant connection from the admin UI: checks the base URL + resolved token
-/// against <c>GET /api/</c>, and optionally reads one entity's state. Confirms the app can actually
-/// read HA before a source is relied upon (SDD §6.2). Token is resolved by reference (env var).
+/// Verifies a Home Assistant connection from the admin UI: checks the base URL + token against
+/// <c>GET /api/</c>, and optionally reads one entity's state. Confirms the app can actually read HA
+/// before a source is relied upon (SDD §6.2).
 /// </summary>
+/// <remarks>
+/// Takes the token already resolved rather than a reference to one. The admin UI must be able to
+/// test a token that has been typed but not yet saved (so not yet encrypted), and keeping the two
+/// storage forms out of here leaves one resolution path in <see cref="EndpointSecret"/>.
+/// </remarks>
 public sealed class HaConnectionTester(IHttpClientFactory httpClientFactory, ILogger<HaConnectionTester> logger)
 {
     private readonly IHttpClientFactory _httpClientFactory = httpClientFactory;
     private readonly ILogger<HaConnectionTester> _logger = logger;
 
     public async Task<HaTestResult> TestAsync(
-        string? baseUrl, string? tokenEnv, string? entityId, CancellationToken cancellationToken = default)
+        string? baseUrl, string? token, string? entityId, CancellationToken cancellationToken = default)
     {
         if (string.IsNullOrWhiteSpace(baseUrl))
         {
             return new HaTestResult(false, "Base URL is required.");
         }
 
-        if (string.IsNullOrWhiteSpace(tokenEnv))
-        {
-            return new HaTestResult(false, "Token env-var name is required (the token is resolved from it at runtime).");
-        }
-
-        var token = Environment.GetEnvironmentVariable(tokenEnv);
         if (string.IsNullOrWhiteSpace(token))
         {
-            return new HaTestResult(false, $"Environment variable '{tokenEnv}' is not set on the server.");
+            return new HaTestResult(false, "No token available to test.");
         }
 
         var client = _httpClientFactory.CreateClient();