fix(renamer): improve episode offset warning messages (#962)

The warning "Episode offset 0 would result in negative episode" was
misleading and caused log spam. The actual issue was either:
1. Parsed episode was 0 or negative (parsing failure or special episode)
2. A negative offset would make a valid episode negative

Changes:
- Differentiate between parsing issues vs offset issues in log messages
- Use debug level for parsed episode issues (likely special episodes)
- Keep warning level only for actual offset problems
- Include original episode value in warning for better debugging
- Handle edge case where parsed episode is 0 by falling back to 1

Fixes #962

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

backend/src/module/manager/renamer.py

@@ -39,12 +39,22 @@ def gen_path(
         season_num = file_info.season
         season = f"0{season_num}" if season_num < 10 else season_num
         # Apply episode offset
-        adjusted_episode = int(file_info.episode) + episode_offset
+        original_episode = int(file_info.episode)
+        adjusted_episode = original_episode + episode_offset
         if adjusted_episode < 1:
-            adjusted_episode = int(file_info.episode)  # Safety: don't go below 1
-            logger.warning(
-                f"[Renamer] Episode offset {episode_offset} would result in negative episode, ignoring"
-            )
+            if original_episode < 1:
+                # Parsed episode is 0 or negative - likely a parsing issue or special episode
+                # Use episode 1 as fallback to avoid invalid filenames
+                adjusted_episode = 1
+                logger.debug(
+                    f"[Renamer] Parsed episode {original_episode} is invalid, using episode 1"
+                )
+            else:
+                # Offset would make episode negative - ignore the offset
+                adjusted_episode = original_episode
+                logger.warning(
+                    f"[Renamer] Episode offset {episode_offset} would make episode {original_episode} negative, ignoring offset"
+                )
         episode = f"0{adjusted_episode}" if adjusted_episode < 10 else adjusted_episode
         if method == "none" or method == "subtitle_none":
             return file_info.media_path
@@ -95,9 +105,10 @@ async def rename_file(
                     ):
                         # Season comes from folder which already has offset applied
                         # Only apply episode offset
-                        adjusted_episode = int(ep.episode) + episode_offset
+                        original_ep = int(ep.episode)
+                        adjusted_episode = original_ep + episode_offset
                         if adjusted_episode < 1:
-                            adjusted_episode = int(ep.episode)
+                            adjusted_episode = 1 if original_ep < 1 else original_ep
                         return Notification(
                             official_title=bangumi_name,
                             season=ep.season,