feat(admin): disable and re-enable customer accounts (#33)

Adds customers.disabled_at, admin disable/enable endpoints, a Status
column and toggle on the Customers tab, and enforcement across every
path that authenticates.

Enforcement lives in attachCustomer, which previously validated only the
session token and its expiry and never read the customer row. Register,
login and password reset all mint sessions, so a single check in the
middleware covers every path rather than three separate ones — and it
means an existing rd_session cookie stops working at once instead of at
its 30-day expiry. Disabling also deletes the sessions outright, so
eviction does not wait for the next request.

Disabling releases the items the customer was holding, in the same
transaction. A disabled account cannot check out, so leaving its
reservations would keep one-of-a-kind stock off the storefront for up to
the cart expiry window for no purpose. Guarded on 'reserved' so a sold
item is never resurrected. Re-enabling restores sign-in but does not give
the items back — they may since have sold.

Sign-in returns an explicit 403 rather than a generic credential failure.
That does confirm the address has an account, which sits awkwardly beside
the deliberately non-enumerating reset in #32; the trade was made the
other way because a disabled customer told "invalid email or password"
resets their password, succeeds, is still locked out, and concludes the
site is broken. The check runs only after the password verifies, so it is
not a bulk membership oracle, and /register already reveals existence.

A reset token issued before the disable no longer mints a session, and no
new tokens are issued for a disabled account — while still answering 200,
so that endpoint stays non-enumerating.

Self-service GDPR export and deletion are blocked along with everything
else, so those requests now need servicing by hand. Worth checking the
privacy policy does not promise unconditional self-service.

Also fixes an unrelated bug the e2e run surfaced: the admin inventory
fired a request per keystroke in the price fields with no sequencing, so
an older response could land after a newer one and repaint stale rows.
Only the most recently issued request may now set state.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/migrations/1787000000000_add-customer-disabled-at.js

@@ -0,0 +1,19 @@
+exports.up = (pgm) => {
+  pgm.sql(`
+    -- Nullable timestamp rather than a boolean: disabling is reversible, and
+    -- this records when it happened without a second column.
+    ALTER TABLE customers ADD COLUMN IF NOT EXISTS disabled_at TIMESTAMPTZ;
+
+    -- attachCustomer joins customers on every authenticated request to check
+    -- this, so the lookup wants an index on the active case.
+    CREATE INDEX IF NOT EXISTS customers_disabled_at_idx
+      ON customers (disabled_at) WHERE disabled_at IS NOT NULL;
+  `);
+};
+
+exports.down = (pgm) => {
+  pgm.sql(`
+    DROP INDEX IF EXISTS customers_disabled_at_idx;
+    ALTER TABLE customers DROP COLUMN IF EXISTS disabled_at;
+  `);
+};