Add campaign layer, GPU model caching, file browser cleanup

- Campaigns: new table, CRUD API, React context + provider
- Sessions: scoped to campaigns, paths under campaigns/{id}/
- File browser: scoped per campaign, removed copy/paste/autoPlay
- Sidebar: campaign selector dropdown at top
- Transcribe: GPU model cached/released via job counter
- Jobs: status text updates dynamically in real-time
- Auto-redirect: blocked when summarize job is active

backend/app/config.py

@@ -14,6 +14,20 @@ TRANSCRIPT_DIR = DATA_DIR / "transcriptions"
 NOTES_DIR = DATA_DIR / "notes"
 DB_PATH = DATA_DIR / "app.db"
 
+CAMPAIGNS_DIR = DATA_DIR / "campaigns"
+
+def campaign_dir(campaign_id: str) -> Path:
+    return CAMPAIGNS_DIR / campaign_id
+
+def campaign_audio_dir(campaign_id: str) -> Path:
+    return campaign_dir(campaign_id) / "audio"
+
+def campaign_transcript_dir(campaign_id: str) -> Path:
+    return campaign_dir(campaign_id) / "transcriptions"
+
+def campaign_notes_dir(campaign_id: str) -> Path:
+    return campaign_dir(campaign_id) / "notes"
+
 for d in (UPLOAD_DIR, AUDIO_DIR, TRANSCRIPT_DIR, NOTES_DIR):
     d.mkdir(parents=True, exist_ok=True)