Restore helpers.seconds_to_hm, accidentally deleted by the collection-completeness commit

74bb993 ("Add partial-collection completeness to the Collection page",
fixes #976) deleted seconds_to_hm from helpers.py as unrelated collateral
damage, breaking every call site that formats a podcast episode duration
(podcast API/views, stats aggregator, stats_podcast) since it's the shared
formatter 3e77176 consolidated seven duplicate implementations into.

Restoring the original implementation rather than re-duplicating it at
each call site.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: dannyvfilms

src/app/helpers.py

@@ -75,6 +75,18 @@ def minutes_to_hhmm(total_minutes):
     return f"{hours}h {minutes:02d}min"
 
 
+def seconds_to_hm(total_seconds):
+    """Convert a duration in seconds to "1h 9m", or to "46m" under an hour.
+
+    Returns "" for a missing or zero duration.
+    """
+    if not total_seconds:
+        return ""
+    hours = total_seconds // 3600
+    minutes = (total_seconds % 3600) // 60
+    return f"{hours}h {minutes}m" if hours else f"{minutes}m"
+
+
 def has_real_image(image):
     """Return whether the value is a usable artwork URL."""
     return bool(image and image != settings.IMG_NONE)