feat(api): M4.2 — REST surface + pgx Postgres store + OpenAPI 3.1

Full M4.2 deliverable: 16 endpoints (tenants CRUD + lifecycle, catalog, entitlements, API keys with argon2 hashing, audit append + filter), Store interface with pgx-backed Postgres + in-memory parallel implementations exercised by the same eachStore harness, openapi.yaml at 3.1 with kin-openapi contract test. M4.3 adds auth.

Refs: M4.2

internal/store/store.go

@@ -0,0 +1,92 @@
+// Package store hides the persistence layer behind a Store interface.
+// Two implementations: Memory (dev convenience, used when DATABASE_URL is
+// empty) and Postgres (production via pgx). Handlers depend on the
+// interface — never on a concrete type.
+package store
+
+import (
+	"context"
+	"errors"
+	"time"
+)
+
+// Sentinel errors.
+var (
+	ErrNotFound     = errors.New("not found")
+	ErrConflict     = errors.New("conflict")
+	ErrInvalidInput = errors.New("invalid input")
+)
+
+// TenantCreate is the input shape for Store.CreateTenant.
+type TenantCreate struct {
+	Slug       string
+	Name       string
+	Plan       string // optional, defaults to "starter"
+	Kind       string // optional, defaults to "customer"
+	SalesOwner string // optional
+}
+
+// TenantUpdate captures partial mutations. Nil fields are left untouched.
+type TenantUpdate struct {
+	Status        *string
+	Plan          *string
+	ErpCustomerID *string
+	StripeCustID  *string
+	TrialEndsAt   *time.Time
+	ContractStart *time.Time
+	ContractEnd   *time.Time
+	SalesOwner    *string
+}
+
+// APIKeyCreate is the input shape for Store.CreateAPIKey.
+type APIKeyCreate struct {
+	TenantID  string
+	Product   string // empty = applies to all products
+	Name      string
+	Scopes    []string
+	Prefix    string
+	Hash      string // argon2id encoded
+	CreatedBy string
+}
+
+// AuditFilter narrows /v1/audit GET results.
+type AuditFilter struct {
+	TenantID string
+	Product  string
+	ActorID  string
+	Action   string
+	Since    *time.Time
+	Until    *time.Time
+	Limit    int
+	Cursor   int64 // id > Cursor (ascending) is the next page anchor
+}
+
+// Store is the persistence contract. Implementations:
+//   - Memory  — in-process, used when DATABASE_URL is empty (dev convenience).
+//   - Postgres — pgxpool-backed, used in stage + prod.
+type Store interface {
+	// Tenants
+	CreateTenant(ctx context.Context, in TenantCreate) (*Tenant, error)
+	GetTenant(ctx context.Context, id string) (*Tenant, error)
+	GetTenantBySlug(ctx context.Context, slug string) (*Tenant, error)
+	UpdateTenant(ctx context.Context, id string, in TenantUpdate) (*Tenant, error)
+
+	// Entitlements
+	UpsertTenantProduct(ctx context.Context, tp TenantProduct) (*TenantProduct, error)
+	ListTenantProducts(ctx context.Context, tenantID string) ([]TenantProduct, error)
+
+	// API keys
+	CreateAPIKey(ctx context.Context, in APIKeyCreate) (*APIKey, error)
+	FindAPIKeyByPrefix(ctx context.Context, prefix string) (*APIKey, string, error) // returns key + hash
+	TouchAPIKeyUsed(ctx context.Context, id string) error
+	RevokeAPIKey(ctx context.Context, id string) error
+	ListAPIKeys(ctx context.Context, tenantID string) ([]APIKey, error)
+
+	// Audit
+	AppendAudit(ctx context.Context, ev AuditEvent) (*AuditEvent, error)
+	ListAudit(ctx context.Context, f AuditFilter) ([]AuditEvent, int64, error) // returns rows + next cursor (0 = none)
+
+	// Lifecycle
+	Close()
+	Ping(ctx context.Context) error
+}