Align docs with archive semantics; fail loud on non-positive expiry (#43)

- README: rewrite the database-target documentation (target-types
  bullet and the per-webhook databases section) to describe the
  shipped archiving semantics -- separate archive-{webhookID}.db,
  debounced close/reopen for offline archiving, auto-recreate,
  creation-validated optional expiry with prune-on-open, and
  fail-loud delivery on archive write errors -- replacing the
  stale always-successful stub description.
- parseArchiveExpiry now returns an error for set-but-non-positive
  durations ("0s", "-5h") instead of silently defaulting to
  keep-forever, matching ValidateArchiveExpiry at creation time;
  the delivery then fails loudly like any other archive error.
  TestParseArchiveExpiry extended with zero and negative cases.
- databaseTarget type comment: "fire-and-forget" -> "no-retry",
  matching the fail-loud behaviour.

internal/delivery/target_database_test.go

@@ -250,15 +250,18 @@ func TestParseArchiveExpiry(t *testing.T) {
 	t.Parallel()
 
 	cases := []struct {
-		name string
-		in   string
-		want time.Duration
+		name    string
+		in      string
+		want    time.Duration
+		wantErr bool
 	}{
-		{"empty config", "", 0},
-		{"explicit never", `{"expiry":"never"}`, 0},
-		{"empty expiry", `{"expiry":""}`, 0},
-		{"duration", `{"expiry":"1h"}`, time.Hour},
-		{"zero duration", `{"expiry":"0s"}`, 0},
+		{"empty config", "", 0, false},
+		{"explicit never", `{"expiry":"never"}`, 0, false},
+		{"empty expiry", `{"expiry":""}`, 0, false},
+		{"duration", `{"expiry":"1h"}`, time.Hour, false},
+		{"unparseable", `{"expiry":"nonsense"}`, 0, true},
+		{"zero duration", `{"expiry":"0s"}`, 0, true},
+		{"negative duration", `{"expiry":"-5h"}`, 0, true},
 	}
 
 	for _, tc := range cases {
@@ -266,15 +269,16 @@ func TestParseArchiveExpiry(t *testing.T) {
 			t.Parallel()
 
 			got, err := delivery.ExportParseArchiveExpiry(tc.in)
+			if tc.wantErr {
+				require.Error(t, err)
+
+				return
+			}
+
 			require.NoError(t, err)
 			assert.Equal(t, tc.want, got)
 		})
 	}
-
-	_, err := delivery.ExportParseArchiveExpiry(
-		`{"expiry":"nonsense"}`,
-	)
-	require.Error(t, err)
 }
 
 // seedDatabaseTargetDelivery seeds a pending delivery for a