perf: add missing indexes on lead_requests, suppliers, credit_ledger

New migration 0021 adds 7 indexes for columns used in WHERE clauses
across admin list routes and the worker refill handler:
- lead_requests(lead_type) — for all lead-type filters
- lead_requests(lead_type, status) — compound filter in lead queries
- lead_requests(lead_type, verified_at) — refill eligibility queries
- lead_requests(country) — country filter in lead results
- suppliers(tier) — tier filter in supplier admin list
- suppliers(claimed_by) — claimed/unclaimed filter
- credit_ledger(supplier_id) — SUM(delta) balance aggregation

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

web/src/padelnomics/migrations/versions/0021_add_performance_indexes.py

@@ -0,0 +1,41 @@
+"""Add missing indexes identified in performance review.
+
+Columns used in WHERE clauses across admin and worker routes that
+lacked indexes and would cause full table scans as the tables grow.
+"""
+
+
+def up(conn) -> None:
+    # lead_requests: filtered by lead_type, status, verified_at, country
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_lead_requests_lead_type"
+        " ON lead_requests(lead_type)"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_lead_requests_type_status"
+        " ON lead_requests(lead_type, status)"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_lead_requests_type_verified"
+        " ON lead_requests(lead_type, verified_at)"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_lead_requests_country"
+        " ON lead_requests(country)"
+    )
+
+    # suppliers: filtered by tier and claimed_by in admin list + worker refill
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_suppliers_tier"
+        " ON suppliers(tier)"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_suppliers_claimed_by"
+        " ON suppliers(claimed_by)"
+    )
+
+    # credit_ledger: balance queries sum delta grouped by supplier_id
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_credit_ledger_supplier"
+        " ON credit_ledger(supplier_id)"
+    )