fix: fallback to archival head when real-time head is null

[NikhilSharmaWe]

Fixes #83

For hybrid datasets, GET /head and GET /finalized-head forwarded the raw hotblocks response to the client — returning null when hotblocks had no data yet, with no fallback to archival.

Root cause: the if dataset.hotblocks.is_some() branch activated for hybrid datasets and returned immediately, so the archival fallback below it was unreachable.

Fix: detect hybrid datasets with an inner network_id.is_some() check and route them through hybrid_head_response, which uses coalesce_head to prefer the real-time head and fall back to archival on any error (including a null body, which fails deserialization into BlockRef).

Mirrors the existing fallback in stream_from_network.

Only src/http_server.rs is changed.

[NikhilSharmaWe]

@kalabukdima, PTAL. Thanks