feat(articles): expose and filter code centrale (artcentrale)

The code centrale is LaFoir'Fouille's 11-digit product identifier
(prefix 1000x). Only products referenced by the central purchasing
office have one: ~355 800 of 427 700 articles.

The column was already synced from SQL Server and returned by the
detail route via `a.*`, but was absent from the list route and could
not be filtered on.

- list route: select artcentrale, add `artcentrale` (partial match)
  and `has_artcentrale=1|0` query params
- filter matches on prefix AND length 11, so malformed values with a
  valid prefix ('10000', '10000192289P') are excluded
- COALESCE in the predicate: without it, NOT (NULL LIKE '1000%') is
  NULL and the 61 442 NULL rows would vanish from has_artcentrale=0
- normalise '' to null in both routes so they agree
- sync: use safeStr so empty values are stored as NULL, not ''

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API_DOCUMENTATION.md

@@ -32,9 +32,17 @@ GET /api/articles
 | `ean`     | string | EAN / GTIN                           | —      |
 | `codefou` | string | Code fournisseur (partiel)           | —      |
 | `actif`   | `1`/`0`| `1` = actif, `0` = suspendu         | —      |
+| `artcentrale` | string | Code centrale (partiel)          | —      |
+| `has_artcentrale` | `1`/`0` | `1` = référencé centrale, `0` = non référencé | — |
 | `page`    | int    | Numéro de page                       | 1      |
 | `limit`   | int    | Lignes par page (max 500)            | 50     |
 
+> **Code centrale** (`artcentrale`) : identifiant LaFoir'Fouille sur 11 chiffres, préfixe `1000x`
+> (ex. `10000137005`). Seuls les produits référencés par la centrale en possèdent un —
+> environ 355 800 articles sur 427 700. Pour les autres, le champ vaut `null`.
+> `has_artcentrale=1` ne retient que les codes conformes : quelques valeurs parasites
+> (`10000`, `10000192289P`) ont le bon préfixe mais pas la bonne longueur, et sont exclues.
+
 **Réponse** :
 ```json
 {
@@ -54,6 +62,7 @@ GET /api/articles
       "suspendu": null,
       "suividatecreation": "2020-01-15T00:00:00.000Z",
       "suividatemodif": "2024-06-01T00:00:00.000Z",
+      "artcentrale": "10000137005",
       "prix_vente_mini": 15.00,
       "prix_vente_maxi": 25.00,
       "eco_ttc": 0.10,
@@ -85,7 +94,7 @@ GET /api/articles/:id
 **Réponse** :
 ```json
 {
-  "article": { "no_id": 12345, "codein": "...", "libelle1": "...", "pa": 10.50, "..." : "..." },
+  "article": { "no_id": 12345, "codein": "...", "libelle1": "...", "artcentrale": "10000137005", "pa": 10.50, "..." : "..." },
   "gtins": [
     { "gtin": "3760000000001", "preferentiel": 1 }
   ],