Send the build version in the outbound User-Agent (closes #313)

The http and slack targets sent the fixed User-Agent webhooker/1.0. The
delivery engine now takes the version from Globals, the value the web UI
footer shows, and one method builds webhooker/<version> for both
targets. The http target still sets it after the configured headers, so
a configured or inbound User-Agent never reaches the wire.

A test builds the engine through its real constructor with a known
version and checks the header each target sends.

Model: opus-5-5

internal/delivery/engine.go

@@ -14,6 +14,7 @@ import (
 	"go.uber.org/fx"
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/lifecycle"
 	"sneak.berlin/go/webhooker/internal/logger"
 	"sneak.berlin/go/webhooker/internal/metrics"
@@ -146,6 +147,7 @@ type EngineParams struct {
 
 	DB        *database.Database
 	DBManager *database.WebhookDBManager
+	Globals   *globals.Globals
 	Logger    *logger.Logger
 	SSRFGuard *Guard
 }
@@ -167,6 +169,10 @@ type Engine struct {
 	retryCh    chan Task
 	workers    int
 
+	// version is the running build's version, the one the web UI
+	// footer shows. userAgent puts it on every outbound request.
+	version string
+
 	// mtr is the delivery metric set. Production wires the
 	// process-wide one; a test can substitute a set registered on
 	// a private registry so its assertions are not disturbed by
@@ -204,6 +210,7 @@ func New(
 		deliveryCh: make(chan Task, deliveryChannelSize),
 		retryCh:    make(chan Task, retryChannelSize),
 		workers:    defaultWorkers,
+		version:    params.Globals.Version,
 		mtr:        metrics.Default(),
 	}
 
@@ -300,6 +307,13 @@ func (e *Engine) ScheduleRetry(
 	})
 }
 
+// userAgent is the User-Agent header of every http and slack
+// delivery request: the program name and the running build's
+// version.
+func (e *Engine) userAgent() string {
+	return "webhooker/" + e.version
+}
+
 // registerHooks wires the engine's start and stop into the fx
 // lifecycle. The start hook's context is deliberately ignored
 // (see start for why the worker pool must not inherit it); the