Add optional inbound webhook signature verification (closes #67)

A receiver URL was a bare v4 UUID and nothing else: anyone who learned
it could store events and, because inbound headers are forwarded to
targets almost verbatim, choose what the downstream service received.

Entrypoints gain an optional scheme/secret pair. GitHub's
X-Hub-Signature-256 (HMAC-SHA256 hex over the raw body) and GitLab's
X-Gitlab-Token (plain shared token) are supported; both compare with
hmac.Equal. With nothing configured an entrypoint behaves exactly as
before, which is also where every pre-existing row lands after
AutoMigrate adds the columns.

Verification runs after the capped body read and before the first
write, so a rejected request leaves no event row, no delivery row and
no delivery task. A configuration the receiver cannot apply — unknown
scheme, or one half of the pair missing — is refused with a 500 rather
than falling back to unverified.

The secret is credential-bearing and is stored in the clear because
HMAC needs the key itself. It is excluded from JSON, kept out of
templates by a new handlers.EntrypointView projection, and absent from
every log line including the rejection path. The UI sets and rotates
it through one form that never renders the stored value.

internal/handlers/ui_copy_test.go

@@ -13,6 +13,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/delivery"
 	"sneak.berlin/go/webhooker/internal/handlers"
 	"sneak.berlin/go/webhooker/internal/session"
+	"sneak.berlin/go/webhooker/internal/signature"
 )
 
 // Template data keys the page templates read. The handlers package has
@@ -268,12 +269,16 @@ func TestEntrypointCopyButtonIsProgressiveEnhancement(t *testing.T) {
 
 	body := renderPage(t, h, sess, "source_detail.html", map[string]any{
 		dataKeyWebhook: webhook,
-		"Entrypoints":  []database.Entrypoint{entrypoint},
-		// The handler passes delivery.NewTargetViews(targets), never
-		// raw targets, so the test data has to have that same shape.
-		"Targets": delivery.NewTargetViews(nil),
-		"Events":  []database.Event{},
-		"BaseURL": "https://hooks.example.com",
+		// The handler passes projected views, never raw rows — an
+		// entrypoint carries its shared secret and a target its
+		// stored credential — so the test data has that same shape.
+		"Entrypoints": handlers.NewEntrypointViews(
+			[]database.Entrypoint{entrypoint},
+		),
+		"Targets":          delivery.NewTargetViews(nil),
+		"SignatureSchemes": signature.Schemes(),
+		"Events":           []database.Event{},
+		"BaseURL":          "https://hooks.example.com",
 	})
 
 	assert.Contains(