Read form fields from the POST body only (closes #160)

internal/handlers/source_management.go read the target destination
with r.FormValue, which falls back to the URL query string when the
field is absent from the body. So

    POST /source/{id}/targets?url=https://hooks.slack.com/services/T/B/S

created a working target from a value carried on the request line,
where logs, proxies, Referer headers and error trackers record it.
That is the remaining ingress path of the credential-exposure class
the render, delivery-error and log-line paths were each closed for.

Every form read in these handlers is now r.PostFormValue, so no
query-string value can populate stored configuration or be taken as a
credential. The one deliberate query read, `page` on the authenticated
pagination links, is untouched: it uses r.URL.Query().Get already.

The access log no longer carries the query on any branch, so the log
half of the report is already mitigated; the Sentry half is not, and
making the body the only place these fields are read from aims every
credential at Sentry's request context. The SDK attaches the request
to every captured event, and SendDefaultPII=false does not cover all
of what it copies: Scope.SetRequest tees the first 10 KiB of the body
into a buffer that ParseForm then fills, and Scope.ApplyToEvent copies
both that buffer and r.URL.RawQuery into the event with no guard,
before BeforeSend runs.

So the BeforeSend hook replaces the query string and the body with a
marker, drops cookies and the remote-address environment, and reduces
the headers to an allowlist. The body is replaced on every route
rather than filtered by route, and that is a choice rather than a
limitation: sentryhttp's recover path puts the request on the context
it hands to RecoverWithContext, the SDK carries that context through
to BeforeSend as hint.Context, and chi's RoutePattern is reachable
from it. Redacting unconditionally is still the right call. Every
handler reads its fields with PostFormValue, so the body is exactly
where the credentials are; the one route whose body is genuine signal
is the receiver, and that body is already stored on the event and
served from the UI, so a tracker is not where anyone reads it; and an
unconditional rule cannot leak on a route somebody forgets to add to
it, which a route-conditional one can.

The headers need an allowlist because the SDK's own filter removes
four names and passes everything else, including X-Csrf-Token and the
shared secrets senders put on the receiver route. Scheme, host, path,
method and X-Request-Id stay, which is what names the failing route
and ties it to the access log line. Nothing dropped is needed to debug
a CSRF rejection: Origin and Referer are kept, and the TLS decision is
already in the retained URL, whose scheme sentry-go derives from the
same r.TLS-or-X-Forwarded-Proto predicate the CSRF middleware uses to
pick its handler.

Second barrier, for the JSON path that does not exist yet: the fields
that hold a credential are tagged json:"-" so the first handler to
marshal a model cannot serialise one. Target.Config holds the
incoming-webhook URL, APIKey.Key is a bearer token, and Setting.Value
holds the session encryption key. delivery.TargetView remains the
masking barrier for the HTML path, which is unaffected.

internal/database/model_secrets_test.go

@@ -0,0 +1,107 @@
+package database_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// keptField is a non-secret value planted alongside each secret, so
+// the assertions below cannot pass by the model marshalling to nothing.
+const keptField = "keepme"
+
+// marshalModel encodes a model the way a future JSON handler would.
+func marshalModel(t *testing.T, v any) string {
+	t.Helper()
+
+	encoded, err := json.Marshal(v)
+	require.NoError(t, err)
+
+	return string(encoded)
+}
+
+// TestModelsDoNotMarshalTheirSecrets pins the barrier for the JSON
+// path. The /api/v1 route group exists and is empty; delivery's
+// TargetView masks the credential for the HTML path only, so without
+// these tags the first handler that marshals a model serialises the
+// secret with it. Each field below is a live credential:
+//
+//   - Target.Config holds an incoming-webhook URL whose path segments
+//     are the bearer token.
+//   - APIKey.Key is a bearer token outright.
+//   - Setting.Value holds the session encryption key.
+//   - User.Password holds the Argon2 hash, and was already tagged.
+func TestModelsDoNotMarshalTheirSecrets(t *testing.T) {
+	t.Parallel()
+
+	const marker = "QQMODELMARKERQQ"
+
+	cases := []struct {
+		name  string
+		model any
+	}{
+		{
+			name: "target config",
+			model: database.Target{
+				Name:   keptField,
+				Type:   database.TargetTypeSlack,
+				Config: `{"webhookUrl":"https://h/s/` + marker + `"}`,
+			},
+		},
+		{
+			name: "api key",
+			model: database.APIKey{
+				Description: keptField,
+				Key:         marker,
+			},
+		},
+		{
+			name: "setting value",
+			model: database.Setting{
+				Key:   keptField,
+				Value: marker,
+			},
+		},
+		{
+			name: "user password hash",
+			model: database.User{
+				Username: keptField,
+				Password: marker,
+			},
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			encoded := marshalModel(t, tc.model)
+
+			assert.NotContains(t, encoded, marker)
+			assert.Contains(t, encoded, keptField)
+		})
+	}
+}
+
+// TestWebhookMarshalsNoTargetConfig covers the nested case: a webhook
+// marshalled with its targets preloaded must not carry the credential
+// through the association either.
+func TestWebhookMarshalsNoTargetConfig(t *testing.T) {
+	t.Parallel()
+
+	const marker = "QQNESTEDMARKERQQ"
+
+	encoded := marshalModel(t, database.Webhook{
+		Name: keptField,
+		Targets: []database.Target{{
+			Name:   "slack",
+			Config: `{"webhookUrl":"https://h/s/` + marker + `"}`,
+		}},
+	})
+
+	assert.NotContains(t, encoded, marker)
+	assert.Contains(t, encoded, keptField)
+}