feat(ops): OpenTelemetry traces + metrics (RECOMMENDATIONS #5)

ASP.NET Core, outbound HttpClient (Gmail/Ollama), and Npgsql instrumentation with
an OTLP exporter that activates ONLY when Otel:Endpoint /
OTEL_EXPORTER_OTLP_ENDPOINT is configured — zero overhead otherwise. Logs remain
on Serilog. Adds a compose 'observability' profile running grafana/otel-lgtm
(Grafana+Tempo+Prometheus+Loki in one container, loopback :3000) with a
documented one-line enablement. 55/55 tests; vuln + format gates clean.

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

src/InboxIntel.Api/InboxIntel.Api.csproj

@@ -15,6 +15,11 @@
     <PackageReference Include="Asp.Versioning.Mvc" Version="8.1.0" />
     <PackageReference Include="Asp.Versioning.Mvc.ApiExplorer" Version="8.1.0" />
     <PackageReference Include="FluentValidation.AspNetCore" Version="11.3.0" />
+    <PackageReference Include="Npgsql.OpenTelemetry" Version="10.0.3" />
+    <PackageReference Include="OpenTelemetry.Exporter.OpenTelemetryProtocol" Version="1.16.0" />
+    <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.16.0" />
+    <PackageReference Include="OpenTelemetry.Instrumentation.AspNetCore" Version="1.16.0" />
+    <PackageReference Include="OpenTelemetry.Instrumentation.Http" Version="1.16.0" />
     <PackageReference Include="Serilog.AspNetCore" Version="8.0.1" />
     <PackageReference Include="Serilog.Sinks.Console" Version="5.0.1" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="6.6.2" />

src/InboxIntel.Api/Program.cs

@@ -16,6 +16,10 @@ using Microsoft.AspNetCore.HttpOverrides;
 using Microsoft.AspNetCore.RateLimiting;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Options;
+using Npgsql;
+using OpenTelemetry.Metrics;
+using OpenTelemetry.Resources;
+using OpenTelemetry.Trace;
 using Serilog;
 
 var builder = WebApplication.CreateBuilder(args);
@@ -122,6 +126,28 @@ builder.Services.AddAuthentication(options =>
 
 builder.Services.AddAuthorization();
 
+// RECOMMENDATIONS #5: OpenTelemetry traces + metrics (ASP.NET, outbound HTTP, Npgsql).
+// The OTLP exporter only activates when Otel:Endpoint (or the standard
+// OTEL_EXPORTER_OTLP_ENDPOINT env var) is configured — zero overhead otherwise.
+// Logs stay on Serilog. Pair with the compose "observability" profile (grafana/otel-lgtm).
+var otlpEndpoint = builder.Configuration["Otel:Endpoint"]
+    ?? Environment.GetEnvironmentVariable("OTEL_EXPORTER_OTLP_ENDPOINT");
+if (!string.IsNullOrWhiteSpace(otlpEndpoint))
+{
+    builder.Services.AddOpenTelemetry()
+        .ConfigureResource(r => r.AddService("inboxintel-api"))
+        .WithTracing(t => t
+            .AddAspNetCoreInstrumentation()
+            .AddHttpClientInstrumentation()
+            .AddNpgsql()
+            .AddOtlpExporter(o => o.Endpoint = new Uri(otlpEndpoint)))
+        .WithMetrics(m => m
+            .AddAspNetCoreInstrumentation()
+            .AddHttpClientInstrumentation()
+            .AddNpgsqlInstrumentation()
+            .AddOtlpExporter(o => o.Endpoint = new Uri(otlpEndpoint)));
+}
+
 builder.Services.AddApiVersioning(o =>
 {
     o.DefaultApiVersion = new ApiVersion(1, 0);