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/database/settings_registry.go

@@ -82,6 +82,7 @@ var SettingDefaults = []SettingDefault{
 	{Key: "sync_queue_batch_size", Value: "50", Type: SettingTypeInt, Min: "1", Max: "10000", RequiresRestart: true, Category: "sync", Group: "Sync Queue", Description: "Maximum items processed per sync queue flush"},
 	{Key: "worker_pool_size", Value: "3", Type: SettingTypeInt, Min: "1", Max: "100", RequiresRestart: true, Category: "performance", Group: "Worker Pool", Description: "Number of background worker goroutines"},
 	{Key: "worker_queue_cap", Value: "100", Type: SettingTypeInt, Min: "1", Max: "10000", RequiresRestart: true, Category: "performance", Group: "Worker Pool", Description: "Background worker job queue capacity"},
+	{Key: "podcast_poll_interval_seconds", Value: "3600", Type: SettingTypeInt, Min: "300", Max: "604800", Category: "podcasts", Group: "Podcasts", Description: "How often to poll podcast feeds for new episodes (seconds; complete feeds back off)"},
 }
 
 // defaultBy indexes SettingDefaults by key for O(1) lookup.
@@ -188,6 +189,13 @@ func (r *SettingsRegistry) ScanPollInterval() time.Duration {
 	return time.Duration(r.getInt("scan_poll_interval_seconds")) * time.Second
 }
 
+// PodcastPollInterval is the feed-poller tick (live setting; the poller
+// re-reads it every cycle — writes go through the settings API path,
+// AGENTS.md gotcha #2).
+func (r *SettingsRegistry) PodcastPollInterval() time.Duration {
+	return time.Duration(r.getInt("podcast_poll_interval_seconds")) * time.Second
+}
+
 // AutoScanEnabled reports whether auto-scanning is on.
 func (r *SettingsRegistry) AutoScanEnabled() bool { return r.getBool("auto_scan_enabled") }