feat: add license entity, repository, and database migration

Licenses table linked to tenants with JSONB features/limits, Ed25519
signed token storage, and revocation support.

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

src/main/resources/db/migration/V006__create_licenses.sql

@@ -0,0 +1,15 @@
+CREATE TABLE licenses (
+    id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    tenant_id   UUID NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+    tier        VARCHAR(20) NOT NULL,
+    features    JSONB NOT NULL DEFAULT '{}',
+    limits      JSONB NOT NULL DEFAULT '{}',
+    issued_at   TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    expires_at  TIMESTAMPTZ NOT NULL,
+    revoked_at  TIMESTAMPTZ,
+    token       TEXT NOT NULL,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_licenses_tenant_id ON licenses (tenant_id);
+CREATE INDEX idx_licenses_expires_at ON licenses (expires_at);