feat: capture EXPLAIN ANALYZE for slow queries and expose via admin e…

[Tobi-8]

Summary of changes:

1. Migration (migrations/20260630_create_slow_query_plans.sql)

• New slow_query_plans table with columns: id, query, params (JSONB), duration_ms, plan (JSONB), executed_at, created_at
• Indexes on created_at DESC and duration_ms DESC

2. Slow query capture (src/config/database.ts)

• Lowered SLOW_QUERY_THRESHOLD_MS default from 1000ms to 500ms
• Added captureExplainAnalyze() — runs EXPLAIN (ANALYZE, COSTS, VERBOSE, FORMAT JSON) via originalPoolQuery (bypasses the monkey-patch to avoid recursion), then inserts into slow_query_plans
• Fired via setImmediate so it's fully async and adds zero latency to the original query

3. Admin endpoint (src/routes/admin.ts)

• GET /admin/slow-queries — returns last 50 records with requireAdmin + logAdminAction middleware

closes #120

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