Add OpenTelemetry, performance metrics, and broaden logging + tests

Observability:
- New FuchsTelemetry (ActivitySource + Meter) defining business counters
  (invoices/reminders/reports rendered, emails/sms sent+failed, MT940 rows,
  MFR calls) and duration histograms (PDF render, report render, email send).
- Program.cs wires OpenTelemetry tracing (ASP.NET Core, HttpClient, SqlClient,
  app source) and metrics (ASP.NET Core, HttpClient, runtime, app meter).
  OTLP export is enabled only when Fuchs:Telemetry:OtlpEndpoint is set, so a
  missing collector never affects the app; disable via Fuchs:Telemetry:Enabled.

Instrumentation + logging:
- Services (Pdf, Invoice, Reminder, Report, Com, Banking, Widget, MfrFactory)
  now emit spans, record metrics, and log entry/result/timing/errors.
- Added dispatch + key-action logging to the previously silent handlers
  (Banking, Reminder, Reports, Requests).

Tests (137 total, +10):
- ProcessWebComServiceTests with a stub HttpMessageHandler cover success
  (API 200), failure (API 500, invalid email, empty mobile), disabled mode,
  the base64 attachment payload contract, and metric emission via MeterListener.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Fuchs/appsettings.json

@@ -38,6 +38,10 @@
       "AccountId": "",
       "Token": "MANAGED_BY_KEYVAULT",
       "Enabled": false
+    },
+    "Telemetry": {
+      "Enabled": true,
+      "OtlpEndpoint": ""
     }
   }
 }