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.

internal/handlers/podcast.go

@@ -10,6 +10,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"log"
 	"sort"
 
 	"bookhoard/internal/database"
@@ -74,6 +75,7 @@ func (h *PodcastHandler) flipSubscriptionToAll(ctx context.Context, userID, feed
 		AutoDownload:      "all",
 		AutoDownloadCount: pgtype.Int4{}, // all needs no count
 	})
+	log.Printf("[PODCAST] flip: user %s → policy 'all' for feed %s (Download All / add checkbox)", userID, feedID)
 }
 
 func NewPodcastHandler(db *database.Queries, svc *services.PodcastService, worker *services.PodcastDownloadWorker) *PodcastHandler {
@@ -83,9 +85,30 @@ func NewPodcastHandler(db *database.Queries, svc *services.PodcastService, worke
 // ---- request/response shapes ----
 
 type AddPodcastFeedRequest struct {
-	LibraryID   string `json:"library_id" form:"library_id" validate:"required"`
-	URL         string `json:"url" form:"url" validate:"required"`
-	DownloadAll bool   `json:"download_all" form:"download_all"`
+	LibraryID   string   `json:"library_id" form:"library_id" validate:"required"`
+	URL         string   `json:"url" form:"url" validate:"required"`
+	DownloadAll FlexBool `json:"download_all" form:"download_all"`
+}
+
+// FlexBool binds checkbox-style booleans on every wire. echo bool-binds
+// via strconv.ParseBool, which accepts "true"/"1" but REJECTS the
+// browser-native checkbox value "on" — and a bind failure is fatal for
+// the WHOLE request (the add modal would 400 "invalid request" on any
+// form variant that posts "on"). §0 item 2 (2026-10-30).
+//
+// A checkbox only contributes its field when CHECKED, so for form
+// posts presence really is the signal — JSON's native bool binding
+// ("false"→false) is unaffected. Absence on either wire = false.
+type FlexBool bool
+
+func (b *FlexBool) UnmarshalParam(v string) error {
+	switch strings.ToLower(strings.TrimSpace(v)) {
+	case "false", "0", "off", "n", "no":
+		*b = false
+	default:
+		*b = true // "true", "on", "yes", "checked", "1" — or any truthy variant
+	}
+	return nil
 }
 
 type UpdatePodcastSubscriptionRequest struct {
@@ -130,11 +153,11 @@ func podcastErrorResponse(c *echo.Context, err error) error {
 // feed counts are tens, not thousands; no facet SQL needed).
 type podcastFeedSummary struct {
 	Feed
-	SubscribedAt   string `json:"subscribed_at"`
-	EpisodeTotal   int    `json:"episode_total"`
-	DownloadedN    int    `json:"episode_downloaded"`
-	UndownloadedN  int    `json:"episode_undownloaded"`
-	NewEpisodes    bool   `json:"new_episodes"`
+	SubscribedAt  string `json:"subscribed_at"`
+	EpisodeTotal  int    `json:"episode_total"`
+	DownloadedN   int    `json:"episode_downloaded"`
+	UndownloadedN int    `json:"episode_undownloaded"`
+	NewEpisodes   bool   `json:"new_episodes"`
 }
 
 // AddFeed POST /api/podcasts/feeds
@@ -245,10 +268,10 @@ func (h *PodcastHandler) GetFeed(c *echo.Context) error {
 		episodeViews = append(episodeViews, episodeView(&eps[i]))
 	}
 	return c.JSON(http.StatusOK, map[string]interface{}{
-		"feed":            feedWithSubscription(&feed, sub, subFound == nil),
-		"episodes":        episodeViews,
-		"gap_count":       countGaps(episodeViews),
-		"undownloaded":    len(episodeViews) - countDownloaded(episodeViews),
+		"feed":               feedWithSubscription(&feed, sub, subFound == nil),
+		"episodes":           episodeViews,
+		"gap_count":          countGaps(episodeViews),
+		"undownloaded":       len(episodeViews) - countDownloaded(episodeViews),
 		"has_library_access": true, // RLS already gated the row; kept for client convenience
 	})
 }