feat: implement busFactor1Only filter (CM-1220)

Filter packages to only those with a single maintainer.
Uses a correlated subquery backed by the UNIQUE(package_id, maintainer_id)
index on package_maintainers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Umberto Sgueglia <usgueglia@contractor.linuxfoundation.org>

Author: ulemons

backend/src/api/public/v1/packages/listPackages.ts

@@ -21,9 +21,7 @@ const querySchema = z.object({
   pageSize: z.coerce.number().int().min(1).max(MAX_PAGE_SIZE).default(DEFAULT_PAGE_SIZE),
   ecosystem: z.string().trim().optional(),
   lifecycle: z.enum(lifecycleValues).optional(), // TODO: filter not yet implemented in DAL
-  busFactor1Only: booleanQueryParam.refine((v) => !v, {
-    message: 'busFactor1Only filter is not yet implemented',
-  }),
+  busFactor1Only: booleanQueryParam,
   staleOnly: booleanQueryParam,
   unstewardedOnly: booleanQueryParam,
   sortBy: z.enum(['name', 'health', 'impact', 'openVulns']).default('name'),
@@ -53,6 +51,7 @@ export async function listPackages(req: Request, res: Response): Promise<void> {
     ecosystem,
     staleOnly,
     unstewardedOnly,
+    busFactor1Only,
     sortBy: effectiveSortBy,
     sortDir,
   })

services/libs/data-access-layer/src/osspckgs/api.ts

@@ -66,6 +66,7 @@ export interface ListPackagesOptions {
   ecosystem?: string
   staleOnly: boolean
   unstewardedOnly: boolean
+  busFactor1Only: boolean
   sortBy: 'name' | 'impact' | 'openVulns'
   sortDir: 'asc' | 'desc'
 }
@@ -94,6 +95,13 @@ export async function listPackagesForApi(
     conditions.push(`(s.status = 'unassigned' OR s.id IS NULL)`)
   }
 
+  if (opts.busFactor1Only) {
+    // UNIQUE(package_id, maintainer_id) on package_maintainers covers this lookup
+    conditions.push(
+      `(SELECT COUNT(*) FROM package_maintainers pm WHERE pm.package_id = p.id) = 1`,
+    )
+  }
+
   const where = `WHERE ${conditions.join(' AND ')}`
 
   // health is a v2 field — fall back to name sort