Remove ?token= deletion-token path (#143)

The deletion token was accepted via the ?token= query parameter on both
DELETE /api/pastes/{id} and /redeem, and round-tripped through the paste
URL after creation. URL-carried bearer secrets leak into reverse-proxy
access logs and browser history.

- API: deletion tokens are now accepted only via the Authorization header
  (Bearer/Token/bare); query params are ignored on both endpoints
- Web create flow: token moves to the browser via a short-lived tok_<id>
  HttpOnly cookie instead of the redirect URL; the paste view reads it
  from the cookie, never from ?token=
- Web view: the delete button calls redeem() which takes the token from
  sessionStorage and sends it as an Authorization header
- Tests: correct token in query must be rejected (403/400); header path
  still deletes/redeems; extraction unit cases updated

Fixes #143

internal/api/burn_test.go

@@ -53,8 +53,9 @@ func TestDeletionTokenRedeem(t *testing.T) {
 		t.Fatal("no deletion token in create response")
 	}
 
-	// wrong token
-	req = httptest.NewRequest("DELETE", "/api/pastes/"+created.ID+"/redeem?token=wrong", nil)
+	// wrong token (#143: token goes in the Authorization header, not the URL)
+	req = httptest.NewRequest("DELETE", "/api/pastes/"+created.ID+"/redeem", nil)
+	req.Header.Set("Authorization", "Bearer wrong")
 	rec = httptest.NewRecorder()
 	h.ServeHTTP(rec, req)
 	if rec.Code != 403 {
@@ -62,7 +63,8 @@ func TestDeletionTokenRedeem(t *testing.T) {
 	}
 
 	// right token: hard delete
-	req = httptest.NewRequest("DELETE", "/api/pastes/"+created.ID+"/redeem?token="+created.DeletionToken, nil)
+	req = httptest.NewRequest("DELETE", "/api/pastes/"+created.ID+"/redeem", nil)
+	req.Header.Set("Authorization", "Bearer "+created.DeletionToken)
 	rec = httptest.NewRecorder()
 	h.ServeHTTP(rec, req)
 	if rec.Code != 200 {