Session counter drifts upward over time

[Mzack9999]

Interactsh version:

v1.3.1 (current main)

Current Behavior:

The sessions metric drifts upward on long-running servers and doesn't match the real number of active clients. Three small bugs add up:

• Register increments the counter before SetIDPublicKey runs, so duplicate ids or bad keys leak +1.
• Deregister decrements before the body is decoded, so garbage POSTs or wrong-secret requests leak -1.
• Cache TTL/size eviction silently removes sessions without ever decrementing.

Expected Behavior:

sessions should track the real number of active sessions: failed register/deregister calls don't move it, and eviction decrements it.

Fixed by #1348.