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/routers/campaigns.py

@@ -0,0 +1,47 @@
+from fastapi import APIRouter, HTTPException
+
+from .. import database as db
+
+router = APIRouter(prefix="/api/campaigns", tags=["campaigns"])
+
+
+@router.get("")
+def list_campaigns():
+    return db.list_campaigns()
+
+
+@router.post("")
+def create_campaign(body: dict):
+    name = body.get("name", "").strip()
+    if not name:
+        raise HTTPException(400, "Name is required")
+    desc = body.get("description", "").strip()
+    return db.create_campaign(name, desc)
+
+
+@router.get("/{campaign_id}")
+def get_campaign(campaign_id: str):
+    campaign = db.get_campaign(campaign_id)
+    if not campaign:
+        raise HTTPException(404, "Campaign not found")
+    return campaign
+
+
+@router.put("/{campaign_id}")
+def update_campaign(campaign_id: str, body: dict):
+    if not db.get_campaign(campaign_id):
+        raise HTTPException(404, "Campaign not found")
+    updated = db.update_campaign(
+        campaign_id,
+        name=body.get("name"),
+        description=body.get("description"),
+    )
+    return updated
+
+
+@router.delete("/{campaign_id}")
+def delete_campaign(campaign_id: str):
+    if not db.get_campaign(campaign_id):
+        raise HTTPException(404, "Campaign not found")
+    db.delete_campaign(campaign_id)
+    return {"ok": True}