AO3-style tag browsing: taxonomies, display, Hardcover sync (schema unverified)

Four flat taxonomies attached to product (bsc_genre, bsc_mood,
bsc_content_warning, bsc_tag) reuse WordPress's native taxonomy archive
system for the "click a tag, see everything with it" browsing AO3 is known
for — no custom archive templates or query logic needed. Verified: all
four register correctly, render as clickable chips on the product page
(content warnings get a distinct notice instead of just another chip),
and the archive page + term count both work end-to-end with real test
data.

HardcoverAdapter + `wp bookstore sync-hardcover-tags` pull genre/mood/
content-warning/freeform tags from Hardcover's API to populate these.
This part is explicitly NOT verified against a live response — Hardcover's
API is in beta with informal docs, and there was no API token available
to confirm the exact query/response shape. Flagged clearly in the adapter
itself; needs a real token + introspection query before trusting the
field-parsing logic in production. ISBN-first-then-title/author matching
handles both real future ISBNs and the current Gutenberg-synthetic
catalog's fake ones.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -48,3 +48,7 @@ HELCIM_ACCOUNT_ID=
 
 # --- Marketing ---
 MAILERLITE_API_KEY=
+
+# --- Hardcover (AO3-style tag browsing — genre/mood/content-warning/tag sync) ---
+# Get a token: hardcover.app account settings -> Hardcover API -> New API Key
+HARDCOVER_API_TOKEN=