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.go

@@ -0,0 +1,69 @@
+package tests
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"github.com/lukaszraczylo/llm-testbench/internal/eval"
+	"github.com/lukaszraczylo/llm-testbench/internal/testkit"
+)
+
+// registerDatabasesTests registers every databases-category test. Each
+// subcategory - postgres, redis, sql-tuning - has its own register function
+// and source file (databases_postgres.go, databases_redis.go,
+// databases_sqltuning.go) to keep any one file from growing past a few
+// hundred lines. The orchestrator wires this into the catalog's All()
+// separately; this package intentionally does not register itself in
+// catalog.go so parallel category worktrees stay independently buildable.
+func registerDatabasesTests(r *testkit.Registry) {
+	registerDBPostgresTests(r)
+	registerDBRedisTests(r)
+	registerDBSQLTuningTests(r)
+}
+
+// dbExactAnswer returns an Evaluator awarding full credit when the
+// response, trimmed of whitespace, at most one layer of surrounding quote
+// characters (' , ", or `), and a single trailing sentence-ending period,
+// equals want case-insensitively. Used across the databases category for
+// prompts that force a single short forced-vocabulary answer (e.g. "yes"
+// or "no", "keyset" or "offset"): it accepts every materially-correct form
+// of that answer (bare, quoted, differently-cased, or with trailing
+// punctuation) without loosening the match to accept a wrong answer.
+func dbExactAnswer(want string) eval.Evaluator {
+	return eval.EvaluatorFunc(func(_ context.Context, response string) eval.Score {
+		got := strings.TrimSpace(response)
+		got = strings.Trim(got, "\"'`")
+		got = strings.TrimSuffix(strings.TrimSpace(got), ".")
+		got = strings.TrimSpace(got)
+		wantTrimmed := strings.TrimSpace(want)
+		if strings.EqualFold(got, wantTrimmed) {
+			return eval.Score{Value: 1, Detail: fmt.Sprintf("equals %q", wantTrimmed)}
+		}
+		return eval.Score{Value: 0, Detail: fmt.Sprintf("got %q, want %q", got, wantTrimmed)}
+	})
+}
+
+// dbJSONArrayLength returns an Evaluator awarding full credit when the
+// response's extracted JSON array (see eval.ExtractJSON) has exactly want
+// elements, zero otherwise. Paired with per-index eval.JSONField checks so
+// an answer with extra or missing elements (e.g. forgetting to apply a
+// HAVING filter, so an extra group leaks through) cannot score full credit
+// merely by getting the checked indices right.
+func dbJSONArrayLength(want int) eval.Evaluator {
+	return eval.EvaluatorFunc(func(_ context.Context, response string) eval.Score {
+		raw, err := eval.ExtractJSON(response)
+		if err != nil {
+			return eval.Score{Value: 0, Detail: err.Error()}
+		}
+		var arr []json.RawMessage
+		if err := json.Unmarshal([]byte(raw), &arr); err != nil {
+			return eval.Score{Value: 0, Detail: fmt.Sprintf("invalid JSON array: %v", err)}
+		}
+		if len(arr) == want {
+			return eval.Score{Value: 1, Detail: fmt.Sprintf("array length %d", len(arr))}
+		}
+		return eval.Score{Value: 0, Detail: fmt.Sprintf("array length %d, want %d", len(arr), want)}
+	})
+}