feat(podcasts): feed poller + podcast_poll_interval_seconds setting (task 1.6)

One ticker re-reading its interval from the settings registry each
cycle (live tunable — the cache stays honest because writes go through
the settings API path, AGENTS.md gotcha #2). Per tick: list feeds
(first-admin identity, invariant 12), skip is_complete feeds (back
off; explicit refresh still works), poll each (conditional GET via
stored validators), and apply the subscriber auto-download policy
union after each successful fetch so new episodes queue without
waiting a tick.

Schema/registry: podcast_poll_interval_seconds (default 3600s, floor
300s, category podcasts) — seeded with ON CONFLICT DO NOTHING per the
idempotency rule.

internal/services/podcast_poller.go

@@ -0,0 +1,78 @@
+// Podcast feed poller (plan §7 task 1.6): one periodic worker ticking
+// on the podcast_poll_interval_seconds system setting (re-read each
+// cycle — the settings API path keeps the registry cache honest),
+// polling every non-complete feed and applying subscriber auto-download
+// policies after each successful fetch (D6).
+//
+// RLS: detached work under the first-admin identity — the service's
+// adminCtx supplies it (invariant 12). Back-off: itunes:complete feeds
+// are skipped by the ticker (explicit refresh still works).
+package services
+
+import (
+	"context"
+	"log"
+	"time"
+
+	"bookhoard/internal/database"
+)
+
+type PodcastPoller struct {
+	db       *database.Queries
+	svc      *PodcastService
+	worker   *PodcastDownloadWorker
+	settings *database.SettingsRegistry
+}
+
+func NewPodcastPoller(db *database.Queries, svc *PodcastService, worker *PodcastDownloadWorker, settings *database.SettingsRegistry) *PodcastPoller {
+	return &PodcastPoller{db: db, svc: svc, worker: worker, settings: settings}
+}
+
+// Start launches the poll loop; exits when ctx is cancelled.
+func (p *PodcastPoller) Start(ctx context.Context) {
+	go func() {
+		for {
+			interval := p.settings.PodcastPollInterval()
+			if interval < time.Minute {
+				interval = time.Minute // floor against bad settings rows
+			}
+			select {
+			case <-ctx.Done():
+				return
+			case <-time.After(interval):
+			}
+			p.tick(ctx)
+		}
+	}()
+}
+
+// tick polls every non-complete feed once.
+func (p *PodcastPoller) tick(ctx context.Context) {
+	actx, err := p.svc.adminCtx(ctx)
+	if err != nil {
+		log.Printf("[PODCAST-POLL] no acting identity, skipping tick: %v", err)
+		return
+	}
+	feeds, err := p.db.ListPodcastFeedsForPolling(actx)
+	if err != nil {
+		log.Printf("[PODCAST-POLL] worklist: %v", err)
+		return
+	}
+	for _, feed := range feeds {
+		if ctx.Err() != nil {
+			return
+		}
+		if feed.IsComplete {
+			continue // back off ended feeds (explicit refresh still works)
+		}
+		if err := p.svc.PollFeed(actx, &feed); err != nil {
+			log.Printf("[PODCAST-POLL] %s: %v", feed.Title, err)
+			continue
+		}
+		// Fresh episodes → apply the subscriber policy union (auto-
+		// download without waiting for the next tick).
+		if _, err := p.worker.ApplyAutoPolicies(actx, feed.ID); err != nil {
+			log.Printf("[PODCAST-POLL] auto-download policies for %s: %v", feed.Title, err)
+		}
+	}
+}