Bridge proposals have no proposer field, blocking relayer attribution

[RUKAYAT-CODER]

Background

bft_consensus.rs::create_proposal builds and stores a cross-chain proposal but never records which relayer/caller submitted it:

/// # TODO
/// - Add a proposer field so off-chain indexers can attribute proposals
///   to specific relayers for analytics and accountability.
pub fn create_proposal(env: &Env, message: CrossChainMessage) -> Result<u64, BridgeError> {
    ...
}

Without a stored proposer, the indexer (indexer/) and any off-chain analytics/reputation system have no on-chain way to attribute a proposal to the relayer that submitted it. This blocks per-relayer accountability (e.g., penalizing relayers who repeatedly submit invalid/expiring proposals) and per-relayer analytics on the observability dashboards already built for issue #321.

Implementation Plan

• Add a proposer: Address field to the proposal struct, captured via env.current_contract_address()/caller identity at creation time in create_proposal, with require_auth() on the proposer.
• Emit the proposer in the proposal-created event so the indexer can pick it up without an extra storage read.
• Update indexer models/queries (if applicable) to surface proposer in proposal analytics.
• Add a test confirming the stored proposal and emitted event both carry the correct proposer address.

Acceptance Criteria

• Every created proposal records a proposer address
• The proposer is authenticated (require_auth) at creation time
• Proposal-created events include the proposer
• A test confirms proposer attribution end-to-end