Docs: adopt "peppercorn" terminology (no interface changes)

[zjma]

• Add a Terminology section in README.md clarifying the naming update:
  • "the pepper" (formerly "pepper base"): long-lived provider secret
  • "peppercorn" (formerly "pepper"): one-way derivative used by circuit/APIs
• Update comments to use "peppercorn" while retaining field/type names for compatibility:
  • prover-service/src/input_processing/public_inputs_hash.rs (hash comment)
  • prover-service/src/request_handler/types.rs (RequestInput.pepper doc)
  • circuit/templates/keyless.circom (signal pepper naming note)
• Update the Python tool print label to "Peppercorn" in circuit/tools/input_gen.py.

Notes:

• No code/ABI changes; all field names and Rust types remain pepper to avoid breaking dependents.
• This PR addresses the internal terminology proposal from Slack.

Slack Thread