feat: add tenant entity, repository, and database migration

Tenants table with slug, tier (LOW/MID/HIGH/BUSINESS), status
(PROVISIONING/ACTIVE/SUSPENDED/DELETED), Logto org reference, and
Stripe IDs.

src/main/resources/db/migration/V005__create_tenants.sql

@@ -0,0 +1,17 @@
+CREATE TABLE tenants (
+    id              UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    name            VARCHAR(255) NOT NULL,
+    slug            VARCHAR(100) NOT NULL UNIQUE,
+    tier            VARCHAR(20) NOT NULL DEFAULT 'LOW',
+    status          VARCHAR(20) NOT NULL DEFAULT 'PROVISIONING',
+    logto_org_id    VARCHAR(255),
+    stripe_customer_id     VARCHAR(255),
+    stripe_subscription_id VARCHAR(255),
+    settings        JSONB NOT NULL DEFAULT '{}',
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at      TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_tenants_slug ON tenants (slug);
+CREATE INDEX idx_tenants_status ON tenants (status);
+CREATE INDEX idx_tenants_logto_org_id ON tenants (logto_org_id);