chore: add PostgreSQL migration for scout tables (#44)

Creates scout_opportunity and scout_claim tables with foreign keys,
unique constraint on (opportunity, team), and expires_at index.
Already applied to dev database.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrations/2026-03-04_add_scout_opportunities_claims.sql

@@ -0,0 +1,57 @@
+-- Migration: Add scout_opportunity and scout_claim tables
+-- Date: 2026-03-04
+-- Issue: #44
+-- Purpose: Support the scouting feature where players can scout cards
+--          from other teams' opened packs within a 30-minute window.
+--
+-- Run on dev first, verify with:
+--   SELECT count(*) FROM scout_opportunity;
+--   SELECT count(*) FROM scout_claim;
+--
+-- Rollback: See DROP statements at bottom of file
+
+-- ============================================
+-- FORWARD MIGRATION
+-- ============================================
+
+BEGIN;
+
+CREATE TABLE IF NOT EXISTS scout_opportunity (
+    id SERIAL PRIMARY KEY,
+    pack_id INTEGER REFERENCES pack(id) ON DELETE SET NULL,
+    opener_team_id INTEGER NOT NULL REFERENCES team(id) ON DELETE CASCADE,
+    card_ids VARCHAR(255) NOT NULL,        -- JSON array of card IDs, e.g. "[10, 11, 12]"
+    expires_at BIGINT NOT NULL,            -- Unix ms timestamp, 30 min after creation
+    created BIGINT NOT NULL                -- Unix ms timestamp
+);
+
+CREATE TABLE IF NOT EXISTS scout_claim (
+    id SERIAL PRIMARY KEY,
+    scout_opportunity_id INTEGER NOT NULL REFERENCES scout_opportunity(id) ON DELETE CASCADE,
+    card_id INTEGER NOT NULL REFERENCES card(id) ON DELETE CASCADE,
+    claimed_by_team_id INTEGER NOT NULL REFERENCES team(id) ON DELETE CASCADE,
+    created BIGINT NOT NULL                -- Unix ms timestamp, auto-set on creation
+);
+
+-- Unique constraint: one claim per team per opportunity
+CREATE UNIQUE INDEX IF NOT EXISTS scout_claim_opportunity_team_uniq
+    ON scout_claim (scout_opportunity_id, claimed_by_team_id);
+
+-- Index for the common query: find unclaimed, expired opportunities
+CREATE INDEX IF NOT EXISTS scout_opportunity_expires_at_idx
+    ON scout_opportunity (expires_at);
+
+COMMIT;
+
+-- ============================================
+-- VERIFICATION QUERIES
+-- ============================================
+-- \d scout_opportunity
+-- \d scout_claim
+-- SELECT indexname FROM pg_indexes WHERE tablename IN ('scout_opportunity', 'scout_claim');
+
+-- ============================================
+-- ROLLBACK (if needed)
+-- ============================================
+-- DROP TABLE IF EXISTS scout_claim CASCADE;
+-- DROP TABLE IF EXISTS scout_opportunity CASCADE;