fix(admin): hash-conflict routes served at sane path — resolve buttons were dead

The admin group already sits under the /api prefix, so registering
"/api/admin/hash-conflicts" produced /api/api/admin/hash-conflicts while the
hash-conflicts page's htmx buttons posted to /api/admin/hash-conflicts/:id/
resolve — a 404 that htmx swallows silently (no swap, no error), so duplicate-
content resolution has been broken since the feature shipped (03cb4c78).

Verified live: sane path 401 anon -> 200 with admin token; doubled path 404.
Caller audit found no other consumers (Android app, koplugin, web reader,
tests, Bruno: no references). Docs already describe the sane path and are now
correct without edits.

internal/router/router.go

@@ -409,9 +409,13 @@ func RegisterRoutes(cfg *Config) *handlers.Handler {
 	admin := protected.Group("", handlers.AdminMiddleware)
 	registerScannerRoutes(protected, admin, scannerHandler)
 
-	// Hash conflict routes (admin only)
-	admin.GET("/api/admin/hash-conflicts", cfg.HashConflictsHandler.ListHashConflicts)
-	admin.POST("/api/admin/hash-conflicts/:id/resolve", cfg.HashConflictsHandler.ResolveHashConflict)
+	// Hash conflict routes (admin only). NOTE: `admin` already sits under the
+	// /api group (protected), so these paths must NOT start with /api —
+	// they used to, producing /api/api/admin/hash-conflicts while the admin
+	// page's htmx buttons posted to /api/admin/hash-conflicts/:id/resolve,
+	// which 404'd silently (htmx swaps nothing on error responses).
+	admin.GET("/admin/hash-conflicts", cfg.HashConflictsHandler.ListHashConflicts)
+	admin.POST("/admin/hash-conflicts/:id/resolve", cfg.HashConflictsHandler.ResolveHashConflict)
 
 	return scannerHandler
 }