Expose spending proof context extension in v1 transaction inputs

Closes #264

The problem

The v1 Explorer API does not return the spending proof context extension of transaction inputs, while the node API does (spendingProof.extension). Example from #264: /api/v1/transactions/{id} returns inputs with spendingProof (proof bytes) only, with no way to see the context extension variables the input was spent with.

The data is already indexed: the node_inputs table has an extension JSON NOT NULL column and the Input DB model carries it (extension: Json) — it was simply never exposed in the API model.

The fix

v1/models/InputInfo now includes an extension: Json field (placed next to spendingProof, consistent with the node's spendingProof: { proofBytes, extension } shape), populated from FullInput.input.extension, with the corresponding tapir schema description. All v1 endpoints returning InputInfo (transactions by id, by address, etc.) now include the extension; the OpenAPI docs are derived from the schema, so they update automatically.

Non-breaking: the change only adds a field to the response payload.