frontend: bump bits to 0.1.3.dev0 (subject-filtered user-limit NATS scans)

[peshence]

Points the frontend's bits dependency at 0.1.3.dev0 (bits 0.1.3),
which contains the subject-filtered user-limit NATS scan fix
(ecmwf/bits-broker#59).

Why

With the bits-leases-userlimits KV bucket bloated past the NATS client's
per-subscription buffer, the broker's whole-bucket scans (list_user_slots on
strict-mode admission, and the periodic reclaim sweep) degraded into an
async-nats "slow consumer" / resubscribe storm that flooded logs and stalled
the dispatcher queue. bits 0.1.3 replaces the keys() + per-key entry()
enumeration with a single server-side subject-filtered watch stream, and
bounds the reclaim sweep with a timeout.

Validation

This exact bits code (the 0.1.3.dev0 tag commit is the merged #59 plus the
version bump) was A/B-tested on a test cluster with a 120k-entry bucket: the
previous frontend produced 953 slow-consumer events in a 240s window, the
new build produced 0.

Change

• frontend/Cargo.toml: bits rev = 9bdacb5 → tag = "0.1.3.dev0"
• Cargo.lock: bits 0.1.2 → 0.1.3 (bits-only)

No image bump / deploy in this PR — that's a follow-up once merged.