test(catalog): add databases category with 30 tests

Add a new databases category with three subcategories: postgres,
redis, and sql-tuning. Each subcategory holds 10 self-contained tests
with deterministic evaluators.

Postgres tests cover EXPLAIN interpretation, index choice, N+1
detection, isolation anomalies, deadlocks, replica failover, bloat
diagnosis, pool sizing, partial indexes, and LISTEN/NOTIFY.

Redis tests cover structure choice, TTL eviction, INCR atomicity,
MULTI versus pipelining, and SCAN versus KEYS. They also cover pub/sub
delivery, Lua atomicity, memory estimation, cache stampede mitigation,
and keyspace anti-patterns. The SCAN versus KEYS test uses a
negation-aware evaluator, so a response that warns against KEYS does
not lose credit for mentioning it.

SQL-tuning tests trace query results against an inline orders table:
aggregates, joins, NULL handling, GROUP BY plus HAVING, and window
functions. The unit tests recompute every numeric or JSON answer
independently, with a hand-written loop separate from the production
query-logic helper.

registerDatabasesTests registers the category. A wiring test verifies
it directly, and the code does not touch catalog.go, so the
orchestrator can wire the category in separately during integration.

internal/tests/databases_test.go

@@ -0,0 +1,58 @@
+package tests
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/lukaszraczylo/llm-testbench/internal/testkit"
+)
+
+// TestRegisterDatabasesTests_Wiring checks registerDatabasesTests wires up
+// all three databases subcategories - postgres, redis, sql-tuning - with 10
+// tests each, all categorized "databases", all with the expected ID prefix
+// per subcategory, MaxTokens omitted (0, per the round-2 authoring rule),
+// and no duplicate IDs across the three sibling files (testkit.Registry.
+// Register panics on a duplicate ID, which would fail this test outright).
+func TestRegisterDatabasesTests_Wiring(t *testing.T) {
+	r := testkit.NewRegistry()
+	registerDatabasesTests(r)
+
+	const wantTotal = 30
+	if r.Len() != wantTotal {
+		t.Fatalf("registerDatabasesTests: registry has %d tests, want %d", r.Len(), wantTotal)
+	}
+
+	wantPrefix := map[string]string{
+		"postgres":   "pg-",
+		"redis":      "redis-",
+		"sql-tuning": "sql-",
+	}
+
+	counts := map[string]int{}
+	for _, tc := range r.All() {
+		if tc.Category != "databases" {
+			t.Errorf("test %q: Category = %q, want %q", tc.ID, tc.Category, "databases")
+		}
+		if tc.Prompt == "" {
+			t.Errorf("test %q: Prompt is empty", tc.ID)
+		}
+		if tc.Eval == nil {
+			t.Errorf("test %q: Eval is nil", tc.ID)
+		}
+		if tc.Description == "" {
+			t.Errorf("test %q: Description is empty", tc.ID)
+		}
+		if tc.MaxTokens != 0 {
+			t.Errorf("test %q: MaxTokens = %d, want 0 (omitted)", tc.ID, tc.MaxTokens)
+		}
+		if prefix, ok := wantPrefix[tc.Subcategory]; ok && !strings.HasPrefix(tc.ID, prefix) {
+			t.Errorf("test %q: subcategory %q wants ID prefix %q", tc.ID, tc.Subcategory, prefix)
+		}
+		counts[tc.Subcategory]++
+	}
+	for _, sub := range []string{"postgres", "redis", "sql-tuning"} {
+		if counts[sub] != 10 {
+			t.Errorf("subcategory %q: got %d tests, want 10", sub, counts[sub])
+		}
+	}
+}