[feature] Add automatic session invitations and prechecks

[marwannettour]

Parent epic: #300

Summary

Add automatic session invitation dispatch and target-side eligibility precheck.

Scope

• Send automatic invitations to resolved target instances.
• Include session ID, creator identity, target identity, creator public key info, protocol version, and purpose.
• Reject invitations on the target when creator trust, grant, key fingerprint, protocol, or challenge checks fail.
• Do not send session password or AES key in clear text.

Acceptance criteria

• Automatic invitations can be dispatched to target client instances.
• Target-side prechecks reject invalid or unauthorized invitations before joining.
• Rejection reasons are returned or logged in a structured way.
• Existing password exchange and AES key exchange remain unchanged after eligibility succeeds.
• Unit tests cover accepted invitation, untrusted creator, wrong target, key mismatch, and incompatible protocol.