Remove inbound request signature verification (closes #279)

The receiver verified an optional per-entrypoint HMAC or shared token
before accepting a request. That is removed outright: the entrypoint
UUID in the URL is the authentication secret, and possession of it
authorises submission. This reverses the feature added in fcead5d.

Deletes the internal/signature package, the signature_scheme and
signature_secret columns from Entrypoint along with their accessors,
the per-entrypoint secret form and its POST route, and the scheme
labelling in EntrypointView. Pre-1.0 with no installed base, so the
columns simply stop being written; there is no migration and no
compatibility path.

Header sanitisation goes with it. SanitizeHeaders existed to strip a
scheme's own credential header before the header map was stored and
forwarded; with no configured credential there is nothing to strip, so
the receiver marshals the headers as received.

The receiver's other protections are untouched: the 1 MB body cap, the
per-IP rate limiter, the 410 for a deactivated entrypoint and the 404
for an unknown UUID.

internal/handlers/ui_copy_test.go

@@ -13,7 +13,6 @@ 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
@@ -269,16 +268,15 @@ func TestEntrypointCopyButtonIsProgressiveEnhancement(t *testing.T) {
 
 	body := renderPage(t, h, sess, "source_detail.html", map[string]any{
 		dataKeyWebhook: webhook,
-		// 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.
+		// The handler passes projected views, never raw rows — a
+		// target carries 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",
+		"Targets": delivery.NewTargetViews(nil),
+		"Events":  []database.Event{},
+		"BaseURL": "https://hooks.example.com",
 	})
 
 	assert.Contains(