feat[agent]: add bidding & negotiation database schema (bids table)

supabase/migrations/004_bidding_system.sql

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+-- ============================================================
+-- Migration 004: Bidding & Negotiation System
+-- ============================================================
+
+-- Bids table for freight offers
+CREATE TABLE bids (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    load_id TEXT REFERENCES loads(id) ON DELETE CASCADE,
+    driver_id TEXT REFERENCES portal_users(id) ON DELETE CASCADE,
+    bid_amount NUMERIC(12,2) NOT NULL,
+    notes TEXT,
+    status TEXT DEFAULT 'pending' CHECK (status IN ('pending', 'accepted', 'rejected', 'counter_offer')),
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW()
+);
+
+-- Indexes for fast lookup of bids per load
+CREATE INDEX idx_bids_load_id ON bids(load_id);
+CREATE INDEX idx_bids_driver_id ON bids(driver_id);
+CREATE INDEX idx_bids_status ON bids(status);
+
+-- ============================================================
+-- Audit triggers for Bids
+-- ============================================================
+CREATE TRIGGER trg_bids_updated_at BEFORE UPDATE ON bids
+    FOR EACH ROW EXECUTE FUNCTION update_updated_at();