Add city-only Provider Listings meta box with JSON-safe persistence

[Copilot]

community posts currently store provider_listings as a raw JSON string consumed by WPGraphQL, but editors had no admin UI to maintain it. This change adds an editor-facing meta box for city pages and preserves JSON escape integrity by writing via $wpdb instead of metadata APIs that unslash values.

• Admin UI (city-level only)

  • Registers a Provider Listings meta box on community edit screens only when listing_type = city.
  • Places the box in normal context with high priority.
  • Renders a 20-row JSON textarea prefilled with pretty-printed JSON (JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE) when parseable.

• Editor feedback in-box

  • Displays live byte count and provider count.
  • Adds inline client-side JSON validation on blur/change/input:
    • invalid JSON → red border + inline error
    • valid JSON → clears error state

• Save path and data integrity

  • Adds nonce generation/verification (wp_create_nonce / wp_verify_nonce) and edit_posts capability gating.
  • Validates submitted JSON with json_decode; invalid payloads are not persisted and surface an admin notice:
    • Provider Listings not saved: invalid JSON.
  • Persists valid JSON through direct SQL update to wp_postmeta using $wpdb->update() (with insert fallback when row is missing), avoiding update_post_meta() unslash corruption.

• Plugin wiring

  • Introduces ProviderListingsMetaBox admin component and registers it in plugin bootstrap.

$updated = $wpdb->update(
    $wpdb->postmeta,
    [ 'meta_value' => $json_string ],
    [ 'post_id' => $post_id, 'meta_key' => 'provider_listings' ],
    [ '%s' ],
    [ '%d', '%s' ]
);

[sanruiz]

LGTM

[chatgpt-codex-connector]

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