v1.7.0: piani Basic/Pro/Premium/Annual + quota giornaliera

Pricing model: subscription mensili (Basic 2.99 / Pro 5.99 / Premium 9.99)
piu' Annual 49.90 one-time. Feature gating per piano (Basic = solo audio,
Pro+ = tutto). Quota giornaliera (10/30/150) con reset mezzanotte Europe/Rome.

Server: tabella daily_usage, endpoint /api/usage/consume e /status,
webhook LS gestisce subscription_* events + mappatura variant_id -> plan
via .env (LS_VARIANT_BASIC|PRO|PREMIUM|ANNUAL).

Client: gating tab in sidebar in base ai features del piano, contatore
"X/Y oggi" + bottone Upgrade, modal blocco con CTA verso landing.

server/migrations/0002_plans_and_usage.sql

@@ -0,0 +1,28 @@
+SET NAMES utf8mb4;
+
+ALTER TABLE licenses
+  ADD COLUMN plan VARCHAR(16) NULL AFTER status,
+  ADD COLUMN daily_limit INT UNSIGNED NULL AFTER plan,
+  ADD COLUMN subscription_id VARCHAR(64) NULL AFTER daily_limit,
+  ADD COLUMN current_period_end BIGINT UNSIGNED NULL AFTER subscription_id,
+  ADD COLUMN expires_at BIGINT UNSIGNED NULL AFTER current_period_end;
+
+UPDATE licenses
+   SET plan = 'annual',
+       daily_limit = NULL,
+       expires_at = NULL
+ WHERE plan IS NULL;
+
+ALTER TABLE licenses
+  ADD KEY idx_licenses_plan (plan),
+  ADD KEY idx_licenses_sub (subscription_id);
+
+CREATE TABLE IF NOT EXISTS daily_usage (
+  license_id   INT UNSIGNED NOT NULL,
+  day          CHAR(10) NOT NULL,
+  count        INT UNSIGNED NOT NULL DEFAULT 0,
+  updated_at   BIGINT UNSIGNED NOT NULL,
+  PRIMARY KEY (license_id, day),
+  CONSTRAINT fk_daily_usage_license
+    FOREIGN KEY (license_id) REFERENCES licenses(id) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;