feat(web): transcripts API + repository lookups

Adds get_attacker_transcripts (mirror of artifacts for session_recorded
logs) and get_session_log for sid→shard resolution. New
/api/v1/transcripts/{decky}/{sid}?offset=&limit= pages asciinema events
out of the shared JSONL day-shard via an mtime-keyed byte-offset index
— never scans the whole shard per request. New
/api/v1/attackers/{uuid}/transcripts lists sessions for drilldown. Both
endpoints admin-gated.

decnet/web/router/transcripts/__init__.py

@@ -0,0 +1,6 @@
+from fastapi import APIRouter
+
+from .api_get_transcript import router as transcript_router
+
+transcripts_router = APIRouter()
+transcripts_router.include_router(transcript_router)