feat(server): tenant-registry skeleton boots against dev stack

Minimal Go service: /healthz + /v1/tenants/by-slug/:slug + /v1/tenants/:id with an in-memory store seeded with the acme tenant. Stdlib-only; pgx + JWT validation land in M4.1 follow-up.

migrations/0001_init.up.sql

@@ -0,0 +1,52 @@
+-- Placeholder for the M4.1 schema (see PLATFORM_ARCHITECTURE.md §5c).
+-- The skeleton uses an in-memory store; this file lands the table shape
+-- the real M4.1 PR will use, so the schema review can happen alongside
+-- the rest of the boot scaffolding.
+
+-- enums --------------------------------------------------------------------
+
+CREATE TYPE tenant_status AS ENUM ('trial', 'active', 'frozen', 'archived', 'demo');
+CREATE TYPE tenant_kind   AS ENUM ('customer', 'demo', 'stage', 'internal');
+
+-- tenants ------------------------------------------------------------------
+
+CREATE TABLE tenants (
+    id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    slug        TEXT UNIQUE NOT NULL CHECK (slug ~ '^[a-z0-9-]{2,40}$'),
+    name        TEXT NOT NULL,
+    status      tenant_status NOT NULL DEFAULT 'trial',
+    kind        tenant_kind   NOT NULL DEFAULT 'customer',
+    plan        TEXT NOT NULL DEFAULT 'starter',
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    trial_ends_at TIMESTAMPTZ
+);
+
+CREATE INDEX tenants_status_idx ON tenants (status);
+
+-- tenant ↔ product entitlements -------------------------------------------
+
+CREATE TABLE tenant_products (
+    tenant_id   UUID NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+    product     TEXT NOT NULL,
+    enabled     BOOLEAN NOT NULL DEFAULT TRUE,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (tenant_id, product)
+);
+
+-- audit log (Retraced-shape; PRODUCT_INTEGRATION_SPEC.md §8.4) ------------
+
+CREATE TABLE audit_log (
+    id          BIGSERIAL PRIMARY KEY,
+    tenant_id   UUID REFERENCES tenants(id),
+    actor_id    TEXT,
+    actor_name  TEXT,
+    action      TEXT NOT NULL,
+    target_id   TEXT,
+    target_type TEXT,
+    metadata    JSONB NOT NULL DEFAULT '{}'::jsonb,
+    source_ip   INET,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX audit_log_tenant_idx ON audit_log (tenant_id, created_at DESC);