Fix two more bugs found in a third code-review pass

- SupplierOffer::delete_for_isbn() was the one Catalog method still
  missing a $wpdb failure check. There's no unique constraint on isbn13
  alone (schema only has a composite key on isbn13+status+base_price), so
  SyntheticOfferGenerator's delete-then-insert pattern relied entirely on
  the delete actually succeeding — a silent failure there left the stale
  row in place alongside the new insert, two rows for one ISBN, with
  best_offer_for_isbns() picking whichever was cheaper by chance and
  generate-offers reporting full success throughout. Verified with the
  same forced-failure reproduction as the review that found it (a BEFORE
  DELETE trigger): before this fix, that scenario left 2 rows for the
  ISBN; after, it throws immediately (caught by generate_all()'s existing
  per-ISBN handler) and the row count stays at 1.

- deploy/lib/env.sh's env_get() didn't strip a trailing \r or leading/
  trailing whitespace from extracted values. Consequential specifically
  because of restore.sh's DB drop/recreate (added earlier this session):
  DB_NAME_VALUE feeds directly into DROP DATABASE IF EXISTS/CREATE DATABASE
  for that name — a CRLF-saved or hand-edited-with-trailing-space .env file
  would silently target a different database name than the real one, with
  no error, defeating the "clean restore" guarantee that fix exists for.
  Also mattered for quote-stripping: a trailing \r landing after a closing
  quote made the end-with-quote check silently fail to match, leaking
  literal quote characters into the value. Verified against CRLF-terminated,
  trailing-whitespace, and quoted+CRLF combinations, including a byte-level
  check that no \r survives; confirmed no regression against normal
  LF-terminated unquoted/quoted values via a full deploy.sh dev run.

wp-content/plugins/bookstore-core/includes/Catalog/SupplierOffer.php

@@ -74,6 +74,21 @@ class SupplierOffer
     public static function delete_for_isbn(string $isbn13): void
     {
         global $wpdb;
-        $wpdb->delete(self::table(), ['isbn13' => $isbn13]);
+        $result = $wpdb->delete(self::table(), ['isbn13' => $isbn13]);
+        if ($result === false) {
+            // The one Catalog method still missing this check, and not a
+            // hypothetical gap: there's no unique constraint on isbn13
+            // alone (schema only has a composite KEY on isbn13+status+
+            // base_price), so SyntheticOfferGenerator's delete-then-insert
+            // pattern relies entirely on the delete actually succeeding.
+            // Verified directly (a forced delete failure via a BEFORE
+            // DELETE trigger): a silent no-op here left the stale row in
+            // place alongside the new insert — two rows for one ISBN, with
+            // best_offer_for_isbns() picking whichever was cheaper by
+            // chance, and generate-offers reporting full success the whole
+            // time. Throwing here routes it into generate_all()'s existing
+            // per-ISBN catch instead of a silent duplicate.
+            throw new \RuntimeException('bsc_supplier_offer delete failed: ' . $wpdb->last_error . ' (isbn13: ' . $isbn13 . ')');
+        }
     }
 }