fix(web): include non-pico channel sessions in history API

[Alix-007]

Summary

• include non-pico session keys in GET /api/sessions instead of filtering to pico-only keys
• keep backward compatibility for existing pico short IDs while allowing full non-pico keys for GET /api/sessions/{id} and DELETE /api/sessions/{id}
• add regression tests for non-pico JSONL and legacy JSON session history flows

Fixes #1996.

Verification

• Command: go test ./web/backend/api
• Result: ok github.com/sipeed/picoclaw/web/backend/api (cached)

[KristjanKruusRIA]

Duplicate of #2002

[Alix-007]

Closing this as a duplicate of #2002 to avoid splitting review effort across two PRs for the same issue. If #2002 stalls or its scope changes materially, I can reopen the work on a fresh clean base.