fix(podcasts): download_all binds on every wire — FlexBool + flip log

Reproduced the §0 item 2 suspect: echo v5 bool-binds form values via
strconv.ParseBool, which accepts "true"/"1" but REJECTS the
browser-native <input type=checkbox> value "on" — and a bind failure
kills the WHOLE AddFeed request (400 "invalid request"), not just
the checkbox. The add modal posts urlencoded (htmx), so any
checkbox-serialization that emits "on" made Download All silently
unusable.

Fix: FlexBool (encoding.TextUnmarshaler-style UnmarshalParam) binds
download_all permissively on every wire — "true"/"on"/"yes"/"checked"
=> true, explicit false/0/off => false, ABSENCE (unchecked box on any
wire) => false; JSON bool binding is native and unaffected. Tolerates
the exact wires htmx and browsers emit.

AddFeed's subscription flip now logs "[PODCAST] flip ... <feed> —
user" so the live flip is provable in app logs (user suspected it
never fired).

Integration test TestPodcastDownloadAllFormEncoding pins the urlencoded
wire (what htmx actually sends): true + on both flip the caller's
policy to 'all', an unchecked add leaves the default.

cmd/server/tests/podcast_test.go

@@ -11,6 +11,7 @@ package main
 
 import (
 	"bytes"
+	"net/url"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -733,6 +734,22 @@ func TestPodcastPageSearchEpisodes(t *testing.T) {
 
 // ---- issue #3: Download All flips the CALLER's auto-download to all ----
 
+// podcastAuthedForm posts urlencoded (what htmx actually sends for non-GET —
+// the bug class in §0 item 2: the add modal posts a form, not JSON).
+func podcastAuthedForm(t *testing.T, setup *TestServerSetup, token, method, path string, form url.Values) (int, []byte) {
+	t.Helper()
+	req, _ := http.NewRequest(method, setup.Server.URL+path,
+		strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err := http.DefaultClient.Do(req)
+	require.NoError(t, err)
+	defer func() { _ = resp.Body.Close() }()
+	var buf bytes.Buffer
+	_, _ = buf.ReadFrom(resp.Body)
+	return resp.StatusCode, buf.Bytes()
+}
+
 func TestPodcastDownloadAllPolicyFlip(t *testing.T) {
 	setup := setupTestServer(t)
 	regularID := getRegularUserID(t, setup.DB)
@@ -807,6 +824,84 @@ func TestPodcastDownloadAllPolicyFlip(t *testing.T) {
 	})
 }
 
+// ---- §0 item 2: download_all via the WIRE the add modal uses (urlencoded) ----
+
+func TestPodcastDownloadAllFormEncoding(t *testing.T) {
+	setup := setupTestServer(t)
+	lib, _ := createPodcastsLibrary(t, setup, "test FormEnc Podcasts")
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/feed.xml", func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/rss+xml")
+		_, _ = w.Write([]byte(podcastFixture("FormEnc Feed", map[string]string{"fe-1": "FormEnc One"})))
+	})
+	mux.HandleFunc("/feed2.xml", func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/rss+xml")
+		_, _ = w.Write([]byte(podcastFixture("FormEnc Default Feed", map[string]string{"fd-1": "FormEnc Default One"})))
+	})
+	mux.HandleFunc("/feed3.xml", func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/rss+xml")
+		_, _ = w.Write([]byte(podcastFixture("FormEnc On Feed", map[string]string{"fo-1": "FormEnc On One"})))
+	})
+	srv := swapFeedServer(t, mux.ServeHTTP)
+
+	t.Run("urlencoded_add_with_download_all_true_flips_policy", func(t *testing.T) {
+		form := url.Values{}
+		form.Set("library_id", uuid.UUID(lib.ID.Bytes).String())
+		form.Set("url", srv.URL+"/feed.xml")
+		form.Set("download_all", "true") // what the add modal's value="true" checkbox submits
+		resp, body := podcastAuthedForm(t, setup, setup.RegularToken, "POST", "/api/podcasts/feeds", form)
+		require.Equal(t, http.StatusCreated, resp, string(body))
+
+		ignore := assertPodcastPolicy(t, setup, "FormEnc Feed", srv.URL+"/feed.xml")
+		assert.Equal(t, "all", ignore,
+			"PIN: urlencoded download_all=true binds exactly like JSON — htmx posts forms, not JSON")
+	})
+
+	t.Run("urlencoded_add_with_browser_native_on_also_flips", func(t *testing.T) {
+		form := url.Values{}
+		form.Set("library_id", uuid.UUID(lib.ID.Bytes).String())
+		form.Set("url", srv.URL+"/feed3.xml")
+		form.Set("download_all", "on") // the raw <input type=checkbox> value — echo's ParseBool chokes on it and failed the WHOLE bind (400) before the FlexBool fix
+		resp, body := podcastAuthedForm(t, setup, setup.RegularToken, "POST", "/api/podcasts/feeds", form)
+		require.Equal(t, http.StatusCreated, resp, string(body))
+
+		assert.Equal(t, "all", assertPodcastPolicy(t, setup, "FormEnc On Feed", srv.URL+"/feed3.xml"))
+	})
+
+	t.Run("urlencoded_add_without_download_all_keeps_default", func(t *testing.T) {
+		form := url.Values{}
+		form.Set("library_id", uuid.UUID(lib.ID.Bytes).String())
+		form.Set("url", srv.URL+"/feed2.xml")
+		resp, body := podcastAuthedForm(t, setup, setup.RegularToken, "POST", "/api/podcasts/feeds", form)
+		require.Equal(t, http.StatusCreated, resp, string(body))
+
+		assert.Equal(t, "off", assertPodcastPolicy(t, setup, "FormEnc Default Feed", srv.URL+"/feed2.xml"),
+			"PIN: an UNchecked add must NOT flip the policy (absent field ≠ true)")
+	})
+}
+
+// assertPodcastPolicy returns the regular user's auto_download for the
+// subscribed feed identified by (title, url).
+func assertPodcastPolicy(t *testing.T, setup *TestServerSetup, title, feedURL string) string {
+	t.Helper()
+	regularID := getRegularUserID(t, setup.DB)
+	regularPG := pgtype.UUID{Bytes: regularID, Valid: true}
+	feeds, _ := setup.DB.ListPodcastFeedsForPolling(rls.WithUser(context.Background(), regularPG))
+	var feedID pgtype.UUID
+	for _, f := range feeds {
+		if f.Title == title && f.FeedUrl == feedURL {
+			feedID = f.ID
+		}
+	}
+	require.True(t, feedID.Valid, "subscribed feed %q (%s) must exist", title, feedURL)
+	sub, err := setup.DB.GetPodcastSubscription(rls.WithUser(context.Background(), regularPG), database.GetPodcastSubscriptionParams{
+		UserID: regularPG, FeedID: feedID,
+	})
+	require.NoError(t, err)
+	return sub.AutoDownload
+}
+
 // ---- logout actually signs out (fake-signout bug, 2026-10-30) ----
 
 func TestLogoutClearsSession(t *testing.T) {