feat: env-configurable default ID3 tags for artist, album, genre

DEFAULT_ID3_ARTIST, DEFAULT_ID3_ALBUM, DEFAULT_ID3_GENRE in .env.
Clip-level metadata always takes priority. Env defaults fill in blanks.
Current defaults: Kagamine Rin / Kithara Studio / Hardcore Techno.

Also updated .env.example with all three new vars (commented out).

backend/src/main.py

@@ -177,13 +177,15 @@ async def download_clip(clip_id: str):
             if clip.get("title"):
                 audio.tags["TIT2"] = TIT2(encoding=3, text=clip["title"])
             
-            # Artist
-            if clip.get("artist"):
-                audio.tags["TPE1"] = TPE1(encoding=3, text=clip["artist"])
+            # Artist (clip field > env default)
+            artist = clip.get("artist") or os.environ.get("DEFAULT_ID3_ARTIST")
+            if artist:
+                audio.tags["TPE1"] = TPE1(encoding=3, text=artist)
             
-            # Album
-            if clip.get("album"):
-                audio.tags["TALB"] = TALB(encoding=3, text=clip["album"])
+            # Album (clip field > env default)
+            album = clip.get("album") or os.environ.get("DEFAULT_ID3_ALBUM")
+            if album:
+                audio.tags["TALB"] = TALB(encoding=3, text=album)
             
             # Comment
             if clip.get("comment"):
@@ -197,9 +199,10 @@ async def download_clip(clip_id: str):
             if clip.get("key"):
                 audio.tags["TKEY"] = TKEY(encoding=3, text=clip["key"])
             
-            # Genre
-            if clip.get("genre"):
-                audio.tags["TCON"] = TCON(encoding=3, text=clip["genre"])
+            # Genre (clip field > env default)
+            genre = clip.get("genre") or os.environ.get("DEFAULT_ID3_GENRE")
+            if genre:
+                audio.tags["TCON"] = TCON(encoding=3, text=genre)
             
             # Album art
             if clip.get("artwork_filename"):