feat: add pending RCA proposal consumer and migration

[Maikol]

Add PendingRcaConsumer to read pending RCA proposals from the shared pending_rca_proposals table (populated by indexer-rs), decode SignedRCA bytes via toolshed, and feed decoded proposals into the agent's reconciliation loop for indexing rule creation.

• Sequelize model (read-only, defined after sync to avoid schema ownership)
• PendingRcaConsumer class with getPendingProposals/markAccepted/markRejected
• DecodedRcaProposal interface with all decoded fields
• ensureAgreementRules branches between new RCA path and legacy fallback
• Migration 23 creates pending_rca_proposals table with IF NOT EXISTS guard
• Threading: start.ts -> client.ts -> actions.ts -> allocations.ts -> dips.ts
• Unit tests with real ABI encode/decode round-trips