Expose finalized state SSZ endpoint

[ch4r10t33r]

Summary

• Add /lean/v0/states/finalized endpoint that serves finalized checkpoint state as SSZ octet-stream
• Cache upstream SSZ payload on consensus and expose it via the new endpoint
• Wire Poller and UpstreamManager to capture full SSZ while maintaining existing consensus logic

Test plan

• Run leanpoint against a Zeam node and hit /lean/v0/states/finalized, confirm content-type application/octet-stream and non-zero content-length
• Point Zeam --checkpoint-sync-url at the leanpoint endpoint and ensure checkpoint sync succeeds